When the owner of three small Maryland newspapers applied to the U.S. Postal Service for a second-class mailing permit, she was surprised to learn that her papers wouldn't be allowed to accept advertisements for private lotteries. A notice of a church bingo party could be described only as a "fun night," and pictures of slot machines and decks of cards were prohibited.

When a California firm began advertising in magazines that its electric air purifier could prevent the spread of acquired immune deficiency syndrome (AIDS), inspectors for the U.S. Postal Service hauled it into court on charges of false advertising.

The two examples illustrate one of the oldest, most pervasive, but least publicized roles of the Postal Service: the public's protector against gambling, pornography and snake oil salesmen who use the mails to take advantage of unsuspecting citizens.

Each year, the Postal Service's Consumer Protection Division files more than 300 complaints involving misuse of the mails. Another hundred or so cases are turned over to the Justice Department for criminal prosecution.

But that role has not been without controversy. Postal inspectors are forced to tread close to the line of censorship in some cases, while in others they must judge whether products and remedies are scientifically sound.

The laws restricting use of the mails for gambling purposes are about as old as the Republic itself, and have been updated periodically over the last two centuries by Congress and state legislatures.

Specifically, publications that receive second-class mailing permits are prohibited from running advertisements that promote gambling and from listing prizes and winners. Congress voted a few years ago to exempt state-run lotteries, however.

The restrictions do not specify whether the editorial content of a newspaper is also covered, and there have been no court cases to decide the issue. In fact, most complaints about newspapers running ads for private lottery or bingo events usually come, not from opponents of gambling, but from the newspaper's competitors.

Postal inspectors say they look for three standards when deciding whether an ad violates the antigambling law: whether a prize is offered, whether the game involves chance rather than skill, and whether a participant has to pay or purchase a product in order to play.

Some newspapers complain that the restrictions tend to favor state-run gambling operations or large companies that offer games for which "no purchase is necessary to win."

The ones most affected, they say, are often nonprofit community groups. "It's your volunteer fire departments, it's your churches, it's the Kiwanis, it's the Elks," said Margaret Lotito, who publishes three small newspapers in southern Maryland. "We can run all the Maryland state lottery ads we want to."

Bob Brinkmann, general counsel for the National Newspaper Association, the trade group of small newspapers, most of which are delivered through the mails, said his group commonly receives complaints from members who are battling with postmasters over the antigambling laws. "The primary problem is that of a local postmaster who sees something he doesn't like, and says he won't mail it," Brinkmann said. "A lot of postmasters get overzealous."

Almost all of the complaints deal with ads for lotteries or bingo nights, Brinkmann added, rather than news stories.

John Ventresco, a Postal Service attorney, said one problem is the lack of case law on the issue: no one has raised a serious challenge, so no court has ruled on what the Postal Service's proper role should be. "If someone did some legal research," he said, "they could come up with some pretty good First Amendment arguments" against the laws.

Over the years, the courts have limited the Postal Service's role in regulating pornography, reserving for judges the touchy issue of deciding what is obscene. The Postal Service, which once vigorously pursued dozens of cases of mail-order pornography, now deals with only a handful.

The agency, however, has been forced to expand its fight against worthless mail-order products into new areas.

"It's not snake oil anymore, but now they use the chemical name for the snake oil and throw in some pseudoscience," said Thomas Ziebarth, a senior attorney in the Consumer Protection Division.

"People will bite on the damnedest things," he said. The most common bogus products, he said, are certain kinds of diet pills, followed by baldness cures, wrinkle removers, bust developers and a potpourri of products "to improve your sex life."

"From a swindler's point of view, that's the perfect way to do business," he said. "What man is going to come forward and admit that he's impotent, and that the solution didn't work and he's still impotent? It's just amazing how few people will complain -- they'll just suffer in silence."

Consequently, most cases are generated by the postal inspectors themselves, who screen publications and television shows for ads that accept mail orders. They also try to work their names onto the mailing lists of distributors.

When a product appears suspect, the inspectors will purchase it and submit it to an expert for examination. A formula that purports to "cure" baldness, for example, might be shown to a dermatologist. In deciding that a product is spurious, the expert must determine that "it would be the consensus of informed medical opinion that it would not work," Ziebarth said.

But undertaking an investigation without a public complaint can be embarrassing. In one case, postal inspectors discovered that the only three people who had purchased a suspected product were postal inspectors.

In general, inspectors follow a rule of thumb in deciding when to initiate investigation: "If it sounds too good to be true," Ziebarth said, "it probably is."