WHEN UNCLE SAM GOES to court, odds are the lawyer representing him will come from the Justice Department. Yet, a poll of federal judges last March rated one-third of the lawyers from the Justice Department and one-fourth of the lawyers from U.S. Attorneys' offices as less than adequate.

Moreover, until recently the Justice Department did little to improve the trial ability of its attorneys. A survey of federal lawyers by the President's Reorganization Project showed the Justice Department spends far less than most federal agencies -- under $50 a lawyer -- on training programs. The Federal Deposit Insurance Corp., on the other hand, spends $1,400 per attorney on training programs.

Attorney General Griffin B. Bell is trying to change that.

He has vastly upgraded the Justice Department's Advocacy Institute, tripling the number of lawyers who take its basic trial course and expanding the length of the course from one to three weeks.

The new, expanded course starts today with 40 attorneys from the Justice Department divisions here and from U.S. Attorney' offices across the country.

The course is geared heavily to providing mock trial experience -- extremely important since law schools fail to emphasize that kind of training and many young lawyers come to Justice with little or no experience of their own.

"This is a way of making up for their lack of litigation experience," said Richard Carter, the head of the Advocacy Institute.

The attorneys will focus on six cases specially prepared for the course. They will have a chance to examine and cross-examine witnesses, and then be critiqued by experienced government trial attorneys who make up the faculty.

The culmination of the first two-week segment of the course will come March 15 and 16, when teams of attorneys will try cases before real U.S. judges in real courtrooms.

These mock trials will be made as close to life as possible. Actual expert witnesses from the FBI will testify, and drama students will play the parts of other witnesses.

These witnesses will be coached to leave out important details or get confused on the stand so the student-attorneys can get practice in refreshing a witness' recollection without asking leading questions and in impeaching the testimony of a wavering witness.

There will even be a jury made up of volunteers from local civic groups and schoold.

With an experienced federal judge and two trial attorneys watching the courtroom performances, "the students will be pretty heavily criticized that day," Carter said.

In other mock trials, Carter said, the institute has videotaped the jury deliberations so the young attorneys could see the reaction to their case in the place where it really counts.

Videotaping will not be done this time, though, because the mock trials are being held in the U.S. District Court here, and federal judges in Washington refuse to allow cameras in their courthouse.