`Second Man' is first-rate comedy The Second Man Comedy by S. N. Behrman. Directed by Thomas Gruenewald.

In 1927, ``The Second Man,'' starring Alfred Lunt and Lynn Fontanne, established S. N. Behrman's credentials as a creator of social comedy in the American manner. It was the beginning of a long and illustrious career. Nearly 60 years later, the wit, grace, and perceptiveness of Behrman's first solo effort qualify the comedy as a period piece that has kept its freshness. The strengths derive primarily from what critic Joseph Wood Krutch called Behrman's ``comedy of illumination.'' The playwright's crystal perceptions and scintillating dialogue make ``The Second Man'' emotionally satisfying as well as amusing.

As ``The Second Man'' opens, witty second-rate novelist Clark Storey (Daniel Gerroll) is planning to marry rich, widowed Mrs. Kendall Frayne (Valerie Von Volz). In return for being kept in the luxury to which he aspires, Storey is prepared to serve as constant and amusing companion. Mrs. Frayne is prepared to put up with his self-indulgence. Storey is also, however, the desired romantic object of the much younger Monica Grey (Jane Fleiss). Monica, in turn, has captured the heart of Austin Lowe (Ivar Brogger), a bespectacled scientist whose ardor is equaled by his inarticulateness.

In the case of Storey, the illumination referred to by critic Krutch includes self-illumination. The customarily flippant Storey explains that beneath his exterior persona is a ``second man'' who perceives all of his self-indulgent and opportunistic tendencies. Yet it is clear-eyed realism about the likely course of their relationship that fortifies Storey's determination to resist the infatuated Monica. The course of true love charted by Behrman encounters comic storms and more than one perilous passage before ``The Second Man'' arrives at its quizzical denouement.

Meanwhile, the cast, directed by Thomas Gruenewald at the Hudson Guild Theatre, keeps commendable pace with the evolving situation and deals admirably with Behrman's urbanely civilized conversation piece. Mr. Gerroll projects the self-perception which prevents Storey from becoming merely insufferable. Mr. Brogger is happily cast as the love-stricken innocent. Miss Von Volz's attractive maturity as Mrs. Frayne is a balance for the impetuosity of Miss Fleiss's determined Monica.

Storey's West Side duplex studio in New York City has been handsomely designed (and lighted) by Paul Wonsek to show how comfortably an impecunious writer could live in the 1920s. (That was in the days that a taxi ride could cost a dollar.) Pamela Scofield designed the period costumes. ``The Second Man'' is scheduled to run through March 2. Fresh Horses Play by Larry Ketron. Directed by Dann Florek.

After riding off in a variety of directions, Larry Ketron's ``Fresh Horses'' falters to a wan conclusion, leaving its central characters sadder and perhaps wiser. Ditto the spectator. Twenty-two-year-old Larkin (Craig Sheffer) is a temporary college dropout bent on obtaining an annulment for Jewel (Suzy Amis), the married teen-ager with whom he has been having an affair. One of Larkin's friends and a mysterious interloper have other ideas. By the time Larkin faces the consequences of decisions ``based on sex and sympathy,'' things have grown impossibly messy for him and the hapless Jewel.

Mr. Ketron mingles metaphor and realism in this ruefully comic view of the adolescent young adults. The Southern countryside setting is an abandoned railroad maintenance station. One character calls it a ``choo-choo-graveyard.'' The young men in their amorous pursuits are compared to 19th-century pony-express riders concerned only with ``fresh horses.'' In the play's wildest scene, a coeducational sextet of students pelt each other with marshmallows, trading insults and trivia amid the barrage.

``Fresh Horses'' comes across as a loose-jointed social comedy about American youth culture, with pretensions to something more substantial. Ketron can be a clever dialoguist, and the performance, staged by Dann Florek, does the writing justice. The responsive WPA Theatre cast includes Mark Benninghofen, John Bowman, Alice Haining, Haviland Morris, and Marissa Chibas. Their zest and energy can't save ``Fresh Horses'' from its own problems. Edward T. Gianfrancesco designed the bleak and cluttered ``graveyard'' setting, with lighting by Phil Monat and costumes by Don Newcomb. The production is scheduled to run through March 2.