The posters plastered in front of Orchestra Hall proclaim simply, ''Slatkin's Back.'' That terse statement speaks volumes to music lovers around this city. It says that a man with a formidable musical reputation has come upriver from St. Louis for five weeks to lead this city's musical Sommerfest once again.
In a few years Leonard Slatkin, the diminutive conductor of the St. Louis Symphony, has leaped from relative obscurity into the orchestral limelight. With the possible exception of the Chicago Symphony's Sir Georg Solti, he dominates the podium in the American heartland.
He also typifies, for better or worse, the often mercurial and star-based conducting scene today.
Slatkin has been guest conductor of the so-called ''big five'' American orchestras - Cleveland, Boston, Chicago, Philadelphia, New York - a feat unmatched by any of his countrymen. Also, this season he will do Gershwin's ''Porgy and Bess'' with the Stuttgart Opera, and he will conduct the Berlin Philharmonic in March.
Some musicians in these orchestras raise doubts about his mastery of the great musical literature; but that hasn't stopped him from blazing an impressive trail through the music world.
His own orchestra has been put in the No. 2 slot among American ensembles by Time magazine, which gives him most of the credit for the ensemble's meteoric rise to that position. The Washington Post raved: ''In the galaxy of American conductors, he belongs right up there next to (Leonard) Bernstein and (James) Levine,'' adding that ''his music-making is always full of personality and feeling.''
His sudden prominence in the image-dominated conducting scene seems all the more remarkable because he is cast so utterly against type.
A principal player in one of the orchestras Slatkin conducts complains that ''he just doesn't look impressive enough - he doesn't have that enigmatic charisma.'' Here, ''charisma'' may be read as ''powerful image,'' a commodity singularly lacking in the conductor from St. Louis. In fact, members of the Chicago Symphony Orchestra have facetiously dubbed him ''Brother Juniper.''
Leonard Slatkin looks more like a tennis player than a monk as he towels off in the plush, spacious conductor's office here after a strenuous rehearsal. But he self-deprecatingly acknowledges, ''I'm not the glamorous type: I'm sort of fat.'' An obvious overstatement, which he follows by saying that, after an intensive bout of rehearsing and performing, he loses several pounds.
Meanwhile, an interview with him bounds over such diverse territory as:
* His preoccupation with baseball, which led him to have someone call him every half hour in Israel during a crucial game in the 1982 World Series.
* His great escape in life - ''frivolous movies.''
* His penchant for finding good restaurants in cities where he performs, and his pronouncement that ''food is crucial'' to good living.
Food may be ''crucial,'' but music appears to be everything: He says it absolutely absorbs his life.
Slatkin appears very much absorbed, indeed, later that evening, as he leads the Minnesota through Gerhard Schedl's erratic, slightly postured ''Tango for Orchestra.'' Note by note, phrase for phrase, he is as much in the music as any conductor could be.
His sharp, clear stick technique continues to prevail in Mendelssohn's Piano Concerto No. 1, and as he and the Minnesota thunder through a reading of Dvorak's Ninth Symphony,
''From the New World,'' which carries itself with evident authority (at the expense sometimes of the transparent beauty in the piece).
His gestures are unchoreographed, rooted in straightforward, musical necessities. He doesn't disdain the all-important traffic-cop role of a conductor; but he also busies himself over virtuosity, phrasing, meaning.
During a rehearsal break earlier in the day, one musician observed: ''He is always highly prepared. He uses his time, and the orchestra's, with great efficiency. Right now, he's probably upstairs going over stuff on the piano.''
''The respect he gets from the orchestra,'' says a principal player in his own orchestra, ''comes not from any mystique, but because they know what he can do. Because of his ability with the stick, he's not going to let you down.''
Some musicians in such major orchestras as the Boston Symphony and the Chicago - which have seen a great deal of Mr. Slatkin as a principal guest conductor - take a less enthusiastic view.
A principal player with the Chicago, for instance, credits him with ''terrific knowledge'' and ''pretty good stick control.'' At the same time, this musician observes: ''I can't imagine him doing a great performance.'' Another Chicago performer says, ''His usual performances are very good; but I wouldn't say that there is much of an electrical spark in them.''
These musicians may rate Slatkin well below the really great conductors; but he is, after all, only 40 years old.
What they worry about is that - as happened to so many of his immediate predecessors on the fast track to great international careers - sudden fame, and the rigorous travel schedule that goes with it, may deprive him of the slow seasoning it takes to mature a really special talent.
It's precisely that seasoning process, Slatkin maintains, that keeps him up until 2 or 3 three in the morning studying scores. This preoccupation keeps him very much alone, he says.
''I've found that people in my profession can't reserve our love for one person. Our affection is spread around among a lot of people, who are all dead. And we're supposed to bring them back to life.''
Some musicians complain that he still has much work to do in giving life, as it were, to the most venerable composers' thoughts. He shies away from the bedrock repertoire of Beethoven, Brahms, and Mozart which conductors tend, rightly or wrongly, to be judged by.
Slatkin admits disarmingly that he is not as comfortable as he might be with some classical and pre-classical literature. He says he came late to Mozart, finding that he ''couldn't apply the principles'' of other music to Mozart pieces: ''Almost six years ago, I did a 'Cosi,' and it was just awful. I had no business doing it.''
On the other hand , Bradford Buckley, a St. Louis bassoonist, observes that ''there's a vast repertoire out there for an orchestra, and to expect a conductor to know it all is ridiculous.
''Different periods of music demand different things. Slatkin is very much a product of an American education in the last half of the 20th century. Because of that, he can do contemporary music better than anyone in the world.''
He has built his reputation on a combination of great European/Russian romantic masterpieces and 20th-century works, especially American. This has endeared him to American composers.
''He's the highest-up figure in the American orchestra world'' that composers can rely on ''as a real champion of American music,'' observes composer John Harbison.
Anyone with a phonograph can see for himself just how brightly Slatkin shines in the music he knows.
The best of this listening is embodied in an absolutely luminous account of Prokofiev's Fifth Symphony, which shows Slatkin and his orchestra at an almost unmatched level of intensity and accomplishment.
The worst can be found in a recording of Mozart arias that bears embarrassing testimony to his self-admitted lack of depth with this music.
Somehwere in the middle lies a mixture of offbeat and more mainstream conducting that has convinced critics, audiences, and many musicians that Leonard Slatkin is a genuine force in American music - one who will make a mark on this country's music in decades to come.