A chilly rain was falling recently, as a man climbed nervously into the darkened front seat of a black sedan in a deserted Reston parking lot.

"I'm Number 438. Are you looking for me?" the man asked the driver.

After a moment's conversation, the man behind the wheel produced an envelope and handed over $500 in cash as his anonymous visitor disappeared back into the night.

The mysterious meeting was neither a drug deal nor espionage, but there was a good reason to keep it secret. Number 438 was an informant, one of hundreds of area residents who have profited by sharing their knowledge of crimes with police in a program that admittedly depends on payoffs.

Called "Crime Solvers," the widely publicized program -- in which informants are supposed to help solve a police department's "Crime of the Week" -- is hailed by police officers responsible for its administration in Fairfax, Prince George's and Montgomery countries, the three jurisdictions where it is in effect.

Even some police and others, including the American Civil Liberties Union, say they are troubled by such blatantly mercenary appeals. Dubious law enforcement officers also question whether "Crime Solvers" plays as crucial a role in closing cases as supporters claim.

"Even though the program appeals to money-grubbing instincts, that's life," says Tom Coyle, the Fairfax police officer in charge of that county's program. "And it works."

Whether it works depends on one's interpretation. The departments have gained extensive publicity for the program through weekly press releases, radio announcements and a weekly spot on a Washington television news show. But Fairfax police have yet to solve their first "Crime of the Week" through the one-year-old program. In 11 months, Prince George's police have cracked only one such case.

Yet Fairfax police say the program's widely publicized telephone number has drawn tipsters who have helped them close 84 felony cases, get 20 convictions and recover nearly $100,000 in stolen property. For helping police arrest 33 individuals in those cases, informants received $3,400.

In Montgomery County, which has the area's oldest program and apparently the most successful, police say "Crime Solvers" has helped close 272 felony cases including 11 "Crimes of the Week," make 85 arrests and recover $400,000 in stolen property. For that, the police arranged rewards totaling $17,650 in amounts ranging from $100 to $1,000.

Some police officials and the businessmen who provide the reward money say that is a small price to pay for helping control crime, one of the most serious problems confronting the Washington area. Others are not so sure.

"We have certain reservations about Crime Solvers," says Gary Hankins, a spokesman for the D.C. Police Department, which has yet to adopt the program despite pressure from the television station that airs the program. "The traditional relationship of law enforcement to the community is not to pay people for doing what they should do as their duty. A sense of cooperation has to be fostered through mutual trust, not money."

"These programs turn everybody who wants to . . . into the seller of information," says John Shattuck of the American Civil Liberties Union, who worries that widespread, uncontrolled use of police informants could trigger numerous "full-scale investigations" of innocent people.

Some investigators agree with Fairfax homicide detective Jim Dooley, who finds the program can force officers to "spend a lot of time spinning your wheels" on bad information. "I have a mixed reaction to the program," says Dooley, who adds that he favors the program nonetheless because "it shows the community is interested."

Hankins points out that it is possible to generate interest as well as tips to crack tough cases without the promise of financial gain. That is precisely what the D.C. police department did recently when detectives were stymied by a lack of clues in investigating the mysterious disappearance of Kathleen Boyden, a 32-year-old WMAL radio executive.

District of Columbia police appealed to the media, which publicized accounts of Boyden's disappearance and urged anyone with information to call a special police number. The next morning, a woman called and reported that a car matching the description of Boyden's was parked across the street from her apartment. Boyden's body was discovered inside, and a suspect was arrested a short time later.

"Whenever we have an investigator who's run up against a dead end, especially when there's a continuing danger to the community, we approach the media on a public-service basis about the case, and we traditionally get a tremendous response. It's the same idea a Crime Solvers, but it's not commercialized," Hankins says.

"There's a real difference between people who walk in off the street and offer the police information and encouraging people to turn in their sister or neighbor by holding out rewards," says Shattuck of the ACLU.

Crime Solvers enthusiasts defend the program by saying informants are merely reporting valuable information about crimes. "If you go for a whole year and you only solve one case, then I submit it's worth it," says WJLA-TV anchorman Paul Berry, an early and ardent supporter of Crime Solvers.

He is not alone in his support. Since it was founded four years ago by a former Albuquerque, N.M., police officer, the program has spread to more than 80 communities around the country, where it operates under various names including "Crime Stoppers" and "Silent Witness."

"About half our calls are from estranged wives, ex-boyfriends and angry neighbors who ordinarily wouldn't talk to a police officer," says Fairfax's Coyle. "A lot of serious crimes are committed without witnesses, and the people who know about them are, say, the rapist's bar buddy who gets mad when he gets stiffed for the bar bill and needs money, so he thinks, 'I'll drop a dime [to call police] and fix that jerk.'"

In some cases, Fairfax detectives say, it is they and not anonymous informants who call Crime Solvers to make sure an informant receives a reward. That is what sex squad detective Glenn Powell did in July while investigating the case of a 14-year-old girl who was raped while babysitting in Reston.

While interviewing neighbors, Powell said he questioned an unemployed bricklayer about to be evicted from his apartment. The man told the police officer that a friend might know the rapist's identity. The man demanded money from Powell for the suspect's name.

"I told him that there was money available for information if it proved to be good," said Powell, who called Crime Solvers to report the tip and later that day secured a warrant for the arrest of 21-year-old John Ewell. Ewell now is in jail awaiting sentencing on the rape charge to which he pleaded guilty.

The Crime Solvers board of directors recently voted to pay the informant $600. The programs pay only if the information results in an indictment. "We might have gotten this information without the money, but what's really good is that now there's money to pay informants. Before it was really tough to pay somebody," Powell says.

"He was really pleased and told his friends, 'Hey, the police actually pay off.' In so many cases we need that extra little bit of information and a lot of times people like this really need the money."