In recent years, advertisers have won from the U.S. Supreme Court new freedoms that could hardly have been dreamed of a decade ago. In the term that opens today the justices are set to rule once again on how far the government can go in regulating advertising.

The key issue is whether the guarantees of freedom of speech contained in the First Amendment apply to advertising, what casebooks call "commercial speech."

The confusion began on the eve of World War II, when New York City stopped a small-time promoter from passing out handbills designed to bring paying crowds to an attraction he owned: a Navy submarine. The Supreme Court upheld the ban, explaining that commercial speech was not entitled to the same constitutional protections given "political speech."

Even though Justice William O. Douglas later called the court's ruling "casual, almost offhand," industry for more than 30 years accepted the distinction and made no attempt to challenge advertising restrictions as infringements of the Bill of Rights.

When the old holding was finally attacked, in the mid-1970s, the successful assault did not come from the corporate world. The first legal restriction on advertising to go was a Virginia law banning the advertising of abortion services. The high court recognized that it is almost impossible to sell a product or service without also advocating some value that can be at the heart of a political debate.

Later cases knocked down such constraints as a Virginia ban on advertising prescription drug prices and a rule of the New York Public Service Commission that power companies could not engage in any advertising promoting the use of electricity. The justices have continued to insist that there is, as a 1978 decision put it, a "common-sense distinction between speech proposing a commercial transaction, which occurs in an area traditionally subject to government regulation, and other varieties of speech." But in every case -- unless the advertising has been untruthful or advocated something against the law -- the decision has struck down the curb being examined.

In all of those cases, however, it was a state or local restriction under attack. This term, the justices deal with an advertising ban imposed by the federal government.

A law governing the Postal Service says, "Any unsolicited advertisement of matter which is designed, adapted, or intended for preventing conception is nonmailable matter, shall not be carried or delivered by mail, and shall be disposed of as the Postal Service directs." The exceptions: ads sent to dealers, doctors, and druggists.

Youngs Drug Products Corp., the New Jersey manufacturer of the Trojan line of condoms, ran into that law when it tried mass mailings of both brochures about contraception in general that featured Trojan products and of multi-item fliers that would include Youngs devices among a variety of other drug store products. When the Postal Service refused to accept the ads, the district court here ruled the underlying law unconstitutional. It is that ruling that the justices will review.

(In the New York State electricity advertising case two years ago, the Supreme Court laid out the rules for considering the constitutionality of any ban on advertising. To stop truthful ads, the justices said, a government must prove that it has some substantial and legitimate purpose that can be achieved by the curbs, and that the rules it is promoting are not "more extensive than is necessary" to achieve those goals.)

In the condom case, the district court accepted the arguments for government interference to protect the privacy of individuals in their homes from mailing that is likely to offend some sensitive addressees.

But the law was declared unconstitutional because the judge felt that more limited restrictions could achieve the same purposes. For instance, he found nothing wrong with contraceptives being mentioned in multi-product drug store mailings as long as the ad is "buried in the middle pages of the flier."

Because the federal government is a formidable litigant at the high court Youngs may be fighting an uphill battle.

Five years ago, Bill of Rights considerations led the Supreme Court to strike down New York State curbs on contraceptive advertising that were part of a scheme also restricting the sale and distribution of the devices. The fact that at least four justices see enough difference between that case and the Postal Service rules to merit hearing oral arguments on the Youngs case suggests that the ban on mailing unsolicitated condom ads may be upheld. If that is done on narrow grounds, Gilbert H. Weil, general counsel of the Association of National Advertisers, says, it would not be worrisome.

But the most limited holding is likely to encourage lawmakers to look for other holes in the First Amendment through which they can push ad restrictions.