''KEEP OUT'' is penciled on the battered door. We knock before entering, to make sure no one's inside. The room looks like a junior high school teachers' lounge. It is windowless, with aged chairs, a Cyclops eye of a clock, and a carpet that was once orange. Along one wall stretches a counter that would be perfect for eating lunch - if you moved all the tape recorders.
''I wouldn't let you in if they were listening,'' says Ray Perini, chief of the Suffolk County narcotics bureau.
This, in fact, is a wiretapping lounge. From here, Suffolk County detectives with headphones and Superscope recorders listen in on suspects' phone calls. It's amazing, they claim, how loose-lipped criminals can be.
''They say stuff like, 'You talk, my phone is tapped,' '' says Mr. Perini.
After declining steadily through the late '70s, the use of wiretaps in law enforcement is again on the rise. In 1982 (the last full year for which data is available) court-approved taps were up 22 percent, to 578. Preliminary figures show the numbers continued to climb during last year.
Yet wiretaps and bugs are powerful, potentially dangerous tools. The average tap hears 58 people, both guilty and innocent. ''Videotapping'' with tiny cameras can leave no place to hide.
Are investigators with headphones trampling on constitutional rights?
''Wiretap law is a disaster,'' claims John Shattuck, national legal director of the American Civil Liberties Union (ACLU).
Over 50 years ago, United States Supreme Court Justice Oliver Wendell Holmes called police wiretapping a ''dirty business.'' Even today, 23 states forbid their police departments from using taps and bugs.
But to officials who use it often, electronic eavesdropping is an invaluable weapon in the war against crime.
Take the Federal Bureau of Investigation. Most of the jump in eavesdropping has been caused by the FBI, in its pursuit of drug rings. Fighting the new wave of such criminal enterprises, says FBI chief William Webster, requires greater use of ''sensitive'' techniques.
''I feel very comfortable using wiretaps, if guidelines are carefully supervised,'' Mr. Webster said in a recent interview. ''They have been enormously effective. There's still a kind of carelessness where telephones are concerned.''
In Suffolk County, on the eastern half of Long Island, there are apparently a lot of careless criminals.
Suffolk prosecutors installed more wiretaps in 1982 - 30 of them - than any other US county. According to federal records, about one-third of the 595 conversations these taps intercepted were ''incriminating.''
''You know that scene in the movie 'Annie Hall,' where Woody Allen sneezes and blows $2,000 worth of cocaine all over the floor?'' says Dave Freundlich, Suffolk assistant district attorney. ''I heard that really happen once.''
Often, says Mr. Freundlich, suspects know they are being tapped, and try to disguise the nature of their conversations. But their codes are sometimes less than cryptic.
''One guy will say 'Bring me a tire. A whole tire,' says Freundlich, ''and then the other will ask, 'Do you want the big tire, or the little one?' ''
Long Island's ragged shoreline is a haven for drug smugglers, and narcotics suspects account for most of Suffolk's wiretaps.
Without electronic help, claim county prosecutors, their arrests would reach no higher than street dealers. With taps, they say, they are getting the top dealers in the county - such as Ronald DeConza, convicted in '82 of distributing cocaine.
Assistant district attorney Freundlich insists that Suffolk detectives strictly follow federal wiretap laws. But he admits that the taps are ''a big drain on our resources.''
Indeed, wiretaps are as expensive as a Mercedes sedan. The average cost of a tap (including both equipment and manpower) was $34,000 in '82, according to the administrative office of the United States Courts.
And that, say critics, is a lot to spend for something they consider both a dangerous intrusion on privacy and unnecessary.
''They should yank them all,'' grumbles Herman Schwartz, a law professor at the American University.
Wiretap are powerful vacuums that suck in the words of both criminals and innocent phone users. Between 1977 and 1982, federal taps overheard some 260,000 people - the vast majority of them innocent of any wrongdoing. Officials must turn off their equipment if a chat is not suspicious. But even a few seconds of eavesdropping, say civil liberties advocates, constitutes an invasion of privacy.
''They are inherently intrusive. It's analagous to searching all the apartments in a building, on the grounds that one may have something in it,'' says the ACLU's Mr. Shattuck.
Wiretaps are also sometimes unconstitutional ''fishing expeditions,'' say civil liberties advocates. Officials reason, ''Let's put a wire on this nasty guy and get him for something,'' critics say.
On a less theoretical level, eavesdropping critics argue that the technique is simply ineffective - that prosecutors, after they cast their electronics net, haul in only a few small criminals.
Over the last five years, wiretaps have led to the conviction of an average 900 criminals annually. The majority of these are small-time gamblers and street dealers, says Herman Schwartz of American University.
''If I was a Mafia figure, I would make darn sure never to say anything incriminating over the phone,'' Shattuck adds.
And judges probably don't watch over wiretaps as closely as they should.
The courts are charged with making sure officers follow the wiretap standards set in the Omnibus Crime Act of 1968.
But ''the truth is, on the state level, oversight is difficult to achieve,'' admits a law-enforcement official who asked not to be named. ''How would you like to leaf through hundreds of conversations?''
The technique of wiretapping itself has changed little in the years since the Omnibus Act was passed. ''Wiremen'' still find the phone box near a suspect's home or apartment, take a short wire with clips on each end, and connect the tapped line with one running back to the prosecutor's office.
Detectives then spend eight-hour shifts in bored seclusion, waiting for the tapped phone to ring.
But technology, since 1968, has not been standing still. Officials now use some gadgets the law did not foresee, such as electronic ''pen registers,'' which record numbers dialed, not conversations, allowing police to discover a suspect's contacts.
''Videotapping,'' in which small cameras watch suspects, is a still-small, but particularly Orwellian new development not covered by the law, says one congressional aide.
If your phone is tapped, you can keep your mouth shut. But if there's a camera in your ceiling, there's nothing you can do, short of hiding under the table. A federal district judge, in 1980, called videotapping ''extraordinarily intrusive,'' although he did not throw out camera-obtained evidence.
The use of bugs and phone taps raises difficult questions about both the need for security and rights to privacy.
It is a subject fraught with tensions.
''Electronic surveillance is the only way to get the big guys,'' sums up Michael Goldsmith, counsel to the New York organized-crime task force, ''but its use needs periodic review.''
Next: Impact of technology on privacy.