Ayn Ducao, a second-year student at Harvard Law School, would like to work for the federal government as a prosecutor when she graduates, but fears she won't be able to afford it.

The 24-year-old graduate of the University of Maryland at College Park already has outstanding student loans from Harvard totaling $74,000. "A lot of students are really interested in these jobs but can't afford to take them because of their loans," Ducao said.

Billionaire Samuel J. Heyman, chairman and chief executive of GAF Corp., shares Ducao's concern that not enough law school graduates are able to take jobs with the government.

So Heyman, a Harvard Law School graduate who began his career with the Justice Department before he became a corporate mogul, plans to set up a $5 million fund at Harvard, scheduled to be announced today, as an experiment in helping young lawyers choose public service rather than private practice.

Philip Heymann, a former deputy attorney general and the Harvard Law professor who will administer the program, said he hopes it will stem the steady decline in the numbers of graduates who opt for federal government jobs. In the Kennedy era, when Heymann graduated from Harvard Law, "it seemed like everyone wanted to go into government," he said.

From the late 1960s to the mid-'70s, 40 to 50 students a year out of about 550 were going into government work, according to data kept by Harvard's career placement center. From 1979 through the 1980s, the figure dropped to the teens; by 1992, it had bounced back to about 25; and, in June 1998, it was between seven and 10, Heymann said.

"We're nearing all-time lows," Heymann said. "And I suspect that other top schools are too."

At the National Association for Law Placement, which for 25 years has been keeping track of how many of all new law graduates take jobs with the government, the percentages have declined steadily: from 18.5 percent in 1974 to 13.3 percent in 1998.

One reason for the change: the steady rise in salaries in constant dollars from private law firms compared with a steady decline in constant dollars for government lawyers.

In 1979, the average federal government salary for Harvard Law graduates with four years' experience was about $41,000 in 1997 dollars after taxes and tuition debt, according to internal numbers from the school. The comparable salary at a Boston law firm was about the same, while a New York law firm salary averaged about $56,000.

By 1998, the gap had widened. A government salary was $33,000 using the same criteria; the average pay at a Boston law firm was about $69,000 and at a New York law firm, $99,000.

"Our students aren't rich," said Heymann, who said the average Harvard Law student graduates with an $80,000 debt. "Students worry a lot about the debt load."

Harvard, like most law schools, has a graduated loan forgiveness program for students. Those who go into jobs paying below $30,000 a year have virtually all of their loans forgiven. Those making up to $50,000 may have a portion forgiven.

But Ducao said that students who want to go to work as public defenders or prosecutors quickly top out of the program at a time when their loan payments could be up to $10,000 a year.

Of the 50 law schools across the country that have loan forgiveness programs, Harvard, Stanford and New York University, among others, have some of the most generous, said David Stern, executive director of the National Association for Public Interest Law. "Yet it's not as if the students graduating [Harvard] are going into public interest work when they could."

The new $5 million Harvard program supported by Heyman will offer about 20 students per year help with their student loans if they go into government service. It is scheduled to begin with the graduating class of 2000.

"The federal government has been badly outdistanced . . . particularly if you're paying for debt," said Heymann. "The object is to reverse a very bad trend for government."

The government's personnel chief, Janice Lachance, praised the Harvard initiative. "Public service careers are among the most rewarding, and I welcome any effort or any partnership to promote federal government careers," she said.

Lachance, director of the Office of Personnel Management, said there is widespread agreement "that the federal pay system must be modernized." She has created a special center to recommend long-term solutions while moving forward on short-term fixes, such as recent efforts to ensure that the government can compete to recruit and retain computer specialists.

Heymann said the particulars of the Harvard awards haven't yet been worked out, but he expects that 10 or so students will get all of their student debt paid for the years they work in government, while others will receive honorariums.

"We'll try to build a cadre, like the honor of federal clerkships," Heymann said. "My guess is it will work."

Staff writer Stephen Barr contributed to this report.