INVADING THE INNER sanctums of the nation's law firms has become a favorite occupation for a new breed of journalists writing for legal publications that have taken point blank aim at lawyers and their profession.

But news, (sometimes called gossip) from inside the firms is not all that the legal publications have to offer. They have become a network of information about changes in the law, decision-making, trends, fees, victories and losses.

Little more than a year ago, three legal newspapers -- each with solid financial backing -- were created for the nation's lawyers, who number close to 500,000. Now, those tabloids, Legal Times of Washington, The National Law Journal and The American Lawyer, each claim a healthy circulation (up from zero) and say they have snared their own piece of the bull market for legal news.

Executives of the three publications say their newspapers are not competing with each other. Instead, they each have sought to establish their own identify. Legal Times offers heavy doses of news about Washington Law -- the regulatory agencies, the federal courts, the Justice Department. The National Law Journal favors long, stylish feature stories with a national overview. And The American Lawyer, a monthly that is the flashiest of the three pubications, goes right to the firms, the lawyers, the judges and their business.

"Federal Judge's Gay Baiting Stuns Party Guests" announced one headline in the American Lawyer's August issue over a story about some allegedly inelegant dinner table conversation."The Devil and Stanley Rader" said another headline, with additional lines to explain that the story was about "How a Jewish lawyer from White Plains found Jesus, big bucks, and lots of trouble doing 'the Work' in California."

Co-owned by the publisher Jay L. Kriegel, editor Steven Brill and a giant London-based newspaper conglomerate, The American Lawyer has a circulation of about 18,500. A subscription to the monthly costs $27.50 -- $10 more than the original price of cheaper than the other two newspapers, which publish weekly.

Printed on heavy bond paper and enlivened by big photographs and fancy graphics, The American Lawyer is sometimes called the People magazine of law, a tag that editor Brill rejects.

"It bothers me that because we're writing about a subject that traditionally has not been seen as interesting and because we're making interesting . . . that there's a feeling that means gossip," Brill said in a interview.

He insists he is not the Rona Barrett of the legal business and describes American Lawyer as a "giant clone" of his law column for Esquire magazine.

"I write very complete, long substantive things," Brill said. Indeed, he has made a name for himself by dropping some big bombshells on the business. Washington lawyer Mitchell Rogovin may still be smarting from Brill's hard-hitting and perceptive analysis of Rogovin's performance in the Hamilton Jordon-Studio 54 case.

The American Lawyer has a staff of 16, including five full-time reporters and three fact-checkers, and an operating budget of $1.3 million. "We're not going to make money for two years and we know that," Brill said. But, he added, "we're in it for the long haul."

The National Law Journal, which wants to be known as the Wall Street Journal of Law, claims the highest circulation of all three publications -- 30,000 at $48 per year. Readers include the major Wall Street and Washington law firms, and nine subscriptions go to the U.S. Supreme Court, said James A. Finkelstein, the journal's president.

Finkelstein and his father, New York political figure Jerry A. Finkelstein, run the powerful New York Law Publishing Co., which publishes the daily New York Law Journal. New York Law Publishing put up $1 million to get the National Law Journal off the ground, and this was the first month that the publication made money, James Finkelstein said.

"This year we'll lose a couple of hundred thousand dollars, but we'll also be making money in the next three to four months on a consistent basis," he said.

The National Law Journal, with 11 reporters, prides itself on polished feature stories about a wide range of subjects, from a profile of a prostitute's lawyer to the tale of a town called Sturgeon and its stench after a toxic chemical spill. The Journal also does its share of investigative work (it broke a story on allegations of police brutality in Suffolk county, N.Y.), covers activities in the Supreme Court, carries a column on law schools and another on tax questions, and publishes a regular listing of federal appeals court decisions called "Riding the Circuits."

In Washington, Legal Times has a circulation just under 5,000 and is the most expensive of the three publiations at $125 a year. Backed by the huge New York publishing house of Harcourt Brace Jovanovich, Legal Times began with a "modest investment" and has carefully targeted a small specialty audience of lawyers with a distinctly Washington practice -- the regulatory agencies, securities, antitrust, environmental and energy law and labor relations, according to co-publisher Stephen Glasser.

Glasser spent more than six years at the New York Law Journal (parent to the National Law Journal), as executive vice-president and executive editor, before Harcourt hired him away to start up its law and business publications. Glasser and his wife Lynn are now copublishers of Legal Times which has a nine-member editorial staff, six of whom are lawyers. Glasser said he is very optimistic that the publication will start making money in 1980. Last week, he said, Legal Times published its biggest issue so far -- 56 pages with 25 pages of advertisements. p

Legal Times has done its share to shake up the Washington legal Community. Last September, for example, the well-established firm of Wilkes & Artis was more than dismayed when Legal Times published some sticky details about a split at the firm.

"A lot of people around our place would like to string them up," one lawyer at Wilkes & Artis said. Sensationalizing the internal washing of the laundry doesn't appeal to me, probably because we got caught in the middle of it," this lawyer said.

"It all came out of court papers," was the response from Legal Times editor David Beckwith, who wrote the story.