Neither rain, nor snow, nor dark of night can stop the arrival of junk mail in the nation's mailboxes. But maybe legislation can. And that's what the direct mail advertisers' trade group is afraid of.

For years, senders of the often-dreaded unsolicited mail have had a free hand at mailing applications for coupons and contests, adult games and automobile parts, magazines and Mothers Day gifts.

After months of hearings and investigation, a federal commission issued a report two years ago which concluded that the receipt of unsolicited advertising mail does not stamp out generally regarded rights of privacy, although the commission suggested that, from then on, the mail advertisers should tread lightly. It issued some recommendations for the mail advertisers to follow, including giving their mail recipients the option of refusing to be on their mailing lists.

Since then, however, few companies have complied with the recommendations voluntarily, and some members of the Direct Mail/Marketing Association Inc. fear that the nation has had junk mail up to its doorknobs and is now ready to close its mail slots to them.

Rather than wait for the government's ax to fall on them, the mail advertisers' association is hacking away at its own members, trying to persuade them voluntarily to stop sending the advertising mail unless the recipient wants it.

"We feel the issue will very much heat up in the next few years," said Judy Kaufman, a representative of the organization. "We're trying to get members to straighten up their act. This is an issue that will not go away."

Why won't the companies comply? "I don't think they recognize how critical it is for them," Kaufman said.

Only about 175 of its 2,000 members have agreed to give their customers the option of having their names removed from the lists, Kaufman said.

Last year, Rep. Bill Green (R-N.Y.) introduced legislation that would have curbed the use of unsolicited advertising mail as part of his omnibus privacy package.

Organizations as diverse as Harper's magazine and the Virginia Home for Boys wrote letters pleading with Green's office not to remove their lifeline because the feel they could not survive without unsolicited mail advertising, the spokesman said.

Although Green decided not to reintroduce the legislation this year the junk-mail issue is not dead, the spokesman said. New privacy bills that would include the direct mail issue are being worked on in Green's office. The new legislation would attempt to curb mail advertising without cutting it all out. Eliminating all such mail "would put a lot of people out of work and paralyze many worthy charities," the spokesman said.

On the other hand, by contributing to one charity, sometimes "you're at peril" of being inundated with mailed requests for donations from other sources, he said. The spokesman said one day he contributed to a national enviromenatal fund and soon afterward received mail from "Save the Whale, Save the Seal, Save the Worm. On one day I got three animals."

The spokesman conceded that the congressman sends junk mail, too.

Today's direct mail advertising, the son of the first Montgomery Ward catalogue sent through the mails in 1872, has grown to encompass a family of direct-response advertising by telephone and with coupons or solicitations in magazines, newspapers and on television which require response through the mails or by phone. It is an $82 billion business which accounts for about 12 percent of sales in the country, Kaufman said. The association's 2,000 member firms solicit funds for charities and sell books, magazines, catalogues, foods, industrial products and insurance. They range from the American Express Co. to Ziff-Davis Publishing Co. Anyone with a mailbox has received some direct mail advertising at one time.

More than 25 percent of all book sales, 70 percent of all magazine subscription sales, 18 percent of photo-finishing sales and 11 percent of record and tape sales were made through direct response advertising. In 1977, $13 billion of the $35.2 billion raised by charities was generated through direct mail alone, the association fact book says.

Although their business is almost as old as mail itself, the direct mailers are trying to change with the times. For instance, instead of using demographics to determine who will receive their solicitations, the advertising firms use psychographics, a method for determining not just where a person lives, but how.

"Psychographics operates on the premise that the customer or prospect must be categorized not only demographically, but by tastes, wants, desires and observable purchasing behavior," according to the association's fact book.

"Although the spending power of this new market isn't precisely known, it is significant, if nothing else, it is young," the fact book states. It concludes that "many companies are switching advertising and marketing strategies to appeal to this new market." So psychographics may narrow the amount of advertising mail one receives, matching it with his or her perceived interests, the advertisers say.

The way direct mail works is through the use of lists which are bought from mailing-list brokers who purchase them from government agencies, credit card companies, magazines or other business. These brokers have about 15,000 consumer and industrial lists, some of which are compiled from specialists who study directories, rosters, registrations at trade shows and club memberships. Lists cost between $35 and $50 for 1,000 names, according to the association.

After the compilers get a name, they must provide data such as date of initial purchase, frequency of purchases, dollars spent, date of last transaction, type of products purchased and where the name came from, the association's fact book says. This is used along with psychographics to determine a mail recipients's category, such as outdoor person, college student or new mother, the association said.Just about everybody fits into a least one category.

Some lists are obtained from state government files such as those for automobile registration, according to Gary Bauer of the direct mail and marketing association. "About a dozen" state legislatures have passed or are contemplating laws preventing the lists from being sold, Bauer said.

"Some state legislatures believe there is a problem here that needs addressing," he added.

"You have to look at all the charities like Save the Pelicans or something like that," Bauer said. Without unsolicited mail, "They wouldn't get off the ground."

The Privacy Protection Study Commission was charged by Congress with investigating, among other things, whether persons engaged in interstate commerce who have mailing lists should have the recipient's permission to have his or her name on the list. The commission concluded that mailers should tell their customers, members or donors voluntarily if they rent, sell or transfer lists to other groups, explain to their customers the criteria for their names appearing on such lists, and give their customers an option of being taken off the list. CAPTION: Picture, Gary Bauer with some mailing list notices. By James M. Thresher - The Washington Post