Joseph J. Sisco, former undersecretary of state and a top expert in U.S. Middle East policy, resigned effective today as chancellor of American University, saying he was "absolutely fed up with fund-raising" although some university trustees charged privately that he hadn't done enough of it.

Sisco said in an interview that he was leaving to give full time to his career as a foreign affairs analyst, which includes regular appearances on the Cable News Network and ABC television, and as a consultant and board member for major corporations.

"I'm very pleased with what I did at American University," Sisco said. "I think it's a better school academically and we brought some visibility to the place. . . .But frankly, a lot of what I did was high-level sophisticated begging, and I'm just absolutely fed up with doing any more fund-raising. I've had enough of it. Now it will be up to the president [Richard Berendzen] to carry on. I've tried to be helpful."

Several trustees and other informed sources, who asked not to be identified, said that during the last year Sisco had seldom gone to his office at AU's Ward Circle campus and appeared to make little effort to raise money for the university even though he was reportedly paid about $40,000 a year.

"Joe was pretty invisible," one trustee said. "Why should we keep paying him?"

Another said: "He had no regular responsibilities and no staff except a part-time secretary. He just wasn't visibly present on campus and didn't seem to be doing much for American University."

Several trustees, reportedly unhappy with Sisco's work as a fund-raiser, wanted to ask him to resign, sources said. But the university's board chairman, retired Methodist bishop James K. Mathews, said Sisco took the initiative, telling a group of trustees during a breakfast at the Metropolitan Club last August that he intended to resign. A formal letter of resignation was submitted last month.

Sisco, 61, became president of American University in 1976 with a strong mandate from the trustees to shore up the wobbly finances and slender endowment of the 87-year-old, Methodist-related university. A year ago he moved on to the part-time post of chancellor, a newly created fund-raising job, after he said he wanted to spend more time speaking and writing on foreign affairs.

"We knew full well that [Sisco's job as chancellor] wasn't a permanent arrangement," Mathews said. "I was satisfied with what he had done for the university, but it was pretty clear that he'd pushed his contacts [for fund-raising] as far as he could, and he'd be leaving."

Mathews said the trustees had no plans to name another chancellor.

Sisco said he went regularly to his university office, usually in the morning, and then went downtown for lunch. He said he worked most afternoons at an office on M Street NW where he and his wife, Jean, a former vice president of Woodward & Lothrop, operate a consulting firm. He said he also traveled for corporate board meetings and speeches and took a trip to the Middle East.

"Listen, there are always going to be some people who don't like you," Sisco said. "They don't know what I did. You don't raise any money just sitting in the office."

During Sisco's four years as president and chancellor, private contributions to American University rose from $1 million to $2.7 million a year, but still accounted for just about 5 percent of operating costs. The university built a long-delayed $6 million library, most of it financed by a 30-year mortgage. Endowment rose modestly from $4.8 million to $5.3 million, including a special fund for the business college. But, according to the National Association of College and University Business Officers, this is still far below virtually all private colleges of American's size, including its chief local rivals, Georgetown, with a $65.4 million endowment, and George Washington, with $30.7 million.

As he leaves, Sisco said, $1.5 million is "in the pipeline," to be given to the university "automatically over the next few years."

Irene Pollin, a long-time trustee and member of the search committee that originally recommended Sisco as AU's president, said there "was always a split on the board and a very vocal group that didn't like him and didn't trust him and kept questioning him about how he spent his time.

"I always liked him," Pollin said. "He's an extraordinary guy. But sometimes it was ambiguous about what he was doing.He would speak and travel around and it was hard to define. You couldn't see any immediate result for the university, but it did do a lot in an indirect way. . . . Sure, he used us, but we used him too -- his prestige, his visibility. That's not bad."

Before Sisco came, Pollin said, American University was in "terrible shape," with declining academic standards as well as financial problems. Along with Berendzen, who was AU's provost before succeeding Sisco as president, "[Sisco] got the wheel turning in the other direction, and that's very important," Pollin said.

"I came from a very poor background, a very poor family," Sisco said. "And I wanted to spend some time at American University to pay back for everything that education did for me. I think I've paid it back, and now I'm really into my third career."