WHEN THE NEXT CROP of rookie lawyers comes to town, some 25 years old and fresh from the classroom, Washington's big, moneyed law firms will pay the best of them $30,000 a year -- for starters.

"Astronomical" is how one partner in Washington law firm described salaries for the young associates. "It's a revolution and I don't know where it's going to end," he said.

Big bucks at the starting gate may have their allure, but it's what happens in the future that counts most, warns James M. Kilmer, a Chicago-based legal executive search consultant -- also known as a headhunter. So in this month's issue of the American Bar Association's Student Lawyer Magazine, Kilmer lays out salary charts for associates across the country and offers some irreverent advice on where to make money and keep making it in the legal business.

The battle for the "Brahmins," as Kilmer likes to call the top law school graduates, has pushed two Wall Street firms to offer starting salaries of $33,000 to associates for the 1980 season. A few of Washington's elite firms may fall in line, according to Kilmer's survey.

Of the nation's law school graduates -- there were more than 33,000 in 1978 -- only 5 percent will have the blue-chip credentials for the big-pay firms, Kilmer said in an interview. The next 20 to 25 percent, with above-average academic records from the better law schools, will head to the good, mid-level firms or government jobs, he said. The bottom 60 to 70 percent are the average students, Kilmer said. He calls them the "untouchables."

"Large numbers of this year's graduating class will be unemployed, underemployed, or forced to set up solo practices or leave town," Kilmer wrote in the Student Lawyer.

At the moment, the low starting salary for associates in Washington is $15,000, with an average of $22,000 for new graduates.

Corporate beginners in Washington -- those who toil for the Potomac Electric and Power Co., for example -- start at a low of $12,500 and a high of $21,500, according to Kilmer.

If the graduate eventually becomes a partner in a small Washington tax firm, his (or her) salary could hit $110,000, according to Kilmer. If, instead, the associate aims for the federal bench, earnings are a mere $54,500 and at the U.S. Supreme Court, $72,000. Consider instead a Midwest attorney/agent for professional sports figures -- at $500,000 -- or a Chicago antitrust lawyer at $750,000.

What about the law school faculty members who put their students on the market in the first place? A brochure for the Yale Law School Alumni Fund recently complained that nobody on its faculty earns close to the salary of a federal judge. Kilmer puts the figure at $40,000 for a tenured law professor at an East Coast law school.

And what of Kilmer, 39, the lawyer-turned-headhunter? "It's a very comfortable way to make $50,000 to $80,000 a year," he said in the telephone interview.

Finding out what lawyers earn is Kilmer's hobby as well as his business. But that is not an easy task in a profession that holds salaries sacred. Each year, Kilmer said, he mails 10,000 questionnaires to law firms, banks and corporations for his salary survey. Fewer than 1,000 are returned. From there, Kilmer said, he "gets on the horn," telephoning his private network of former classmates, fraternity brothers, law school buddies and friends with guarantees of confidentiality in exchange for the figures.

The way Kilmer sees it, the old boy network that lends a hand to friend, relatives and classmates who want to become partners is still part of the law business. But the real push these days if for the "rainmakers," the Joe Califanos who bring in the clients. The question a firm asks its lawyers these days is, "What gift do you bear?" Kilmer said.

In law, like other professions, specialties are the future. But Kilmer advises budding lawyers to be aware of areas that are "hot" and those that are not.

The big growth area is a family law and "palimony" -- the Lee and Michelle Marvin variety -- but antitrust law generates "almost obscene salaries," Kilmer says.

Corporations have beefed up their in-house litigation sections because private firmes cost too much; tax lawyers are always in vogue because tax shelters are, according to Kilmer, "the American way"; sophisticated real estate lawyers, familiar with complex financing, are in, but title work "is boring, the pay is bad and only other title companies and banks will hire you," Kilmer wrote. Appellate law is scholarly but unmarketable and there's little demand for pollution lawyers. But white collar crime is "the Ivy League of the criminal class," Kilmer said.

A final thought: Kilmer says Washington pays its lawyers well, but "if you're talking about pretty good early money and a future of making megabucks," the place to go is Houston, for energy and real estate law and deal-making.

For those who want to know whether thy have passed the D.C. Bar before they start dreaming of fat salaries: the Committee on Admissions (the bar examiners) is just about through the last of the exams for the 1,632 candidates who took the bar in July. Last year, the results were posted at the D.C. Court of Appeals the Friday after Thanksgiving, but the announcement is expected later this time around. "I will be soon," was all that one source would say.