Sen. Paul S. Trible Jr. (R-Va.) announced last night that he will not seek a second term, a move that shocked Virginia's Republican Party and fueled speculation that former Democratic governor Charles S. Robb will soon enter next year's Senate race.

Trible, 40, a former three-term House member and prosecutor who serves on the congressional Iran-contra committee, said he was getting out because of frustration with the legislative process and a desire to spend more time with his wife Rosemary and their two children.

"Simply put, the Senate prevents me from seeing enough of Rosemary, Mary Katherine and Paul," Trible said in a two-minute paid announcement aired by Virginia and Washington area television stations.

Neither Trible nor his aides said what the senator plans to do when his term is completed, but he did not rule out another bid for office, such as governor of Virginia. Trible has at least $1.5 million in cash from a yearlong fund-raising effort.

"I'll seek other opportunities to make a difference in the life of our state and nation," he said.

In an interview with The Washington Post last night, Trible said he has "absolutely not" ruled out running for another office, including governor.

"I'm not foreclosing any opportunity because I don't know what the future might hold," he said.

Trible said he could have defeated Robb and contended that the former governor is not really interested in running for the Senate, but is under intense pressure from his successor, Gov. Gerald L. Baliles, and the Democratic Party to do so.

"I don't think he {Robb} would have challenged me for the Senate, and it's less likely he will run now," Trible said. "I don't think he wants to be a senator."

He said he decided to announce his decision now to give the Republicans enough time to field a strong candidate.

Robb said last night that his initial reaction, when told of Trible's announcement, was "Are you serious?" The former governor was attending a political event in Hopewell, Va., when reporters informed him that Trible would not seek a second six-year term in the Senate.

Robb said Trible's announcement does not change his plan, which is to wait until after the November legislative elections before "giving formal consideration" to whether he should make the Senate race.

Virginia Democrats and some political analysts said last night that Trible's decision was triggered in part by recent polls showing that Robb would defeat him handily in a two-way race.

Thomas R. Morris, a political scientist at the University of Richmond, said, "It's very difficult to accept family {considerations} as Senator Trible's entire reason for pulling out, especially given the effort he's already put into the campaign."

However, aides to Trible said late yesterday that he made up his mind last month not to seek a second Senate term, and insisted that the primary consideration was his family.

Sen. Rudy Boschwitz (R-Minn.), chairman of the National Republican Senatorial Committee, said that "every senator understands that the Senate and small children don't mix well. It can be tough on marriages, too. We admire Paul's priorities."

Trible's daughter is 10; his son will be 7 on Saturday.

"There will be a lot of Democrats taking credit for his exit, but the only people who deserve the credit are Paul and Mary Katherine," said Richard Cullen, a Virginia lawyer and close friend of Trible. "It would have been a tough race, everybody knows that, but history favors incumbent senators over popular governors."

Trible's announcement took officials of Virginia's beleaguered Republican Party by surprise and prompted expressions of deep disappointment.

"I understand and respect his decision," said Donald Huffman, GOP state party chairman. "I especially respect Paul's feelings that his family has got to come first. But he has been a successful and popular public servant all his life. I will be extremely sorry to see him leave the U.S. Senate."

Stephen D. Haner, a spokesman for the state GOP, described Trible's announcement as "a big shock."

State Republican strategists, who were asked Friday night to report to party headquarters in Richmond yesterday, said Trible's decision was all the more surprising because he had said he was in no way intimidated by the prospect of being challenged by Robb. "I can tell you one thing: Paul Trible was not scared of Chuck Robb," a senior party official said last night.

Still, there were early indications that the race would have been extremely difficult for Trible. For instance, according to a survey conducted July 23 to 26 by Mason-Dixon Opinion Research, Virginia voters favored Robb 52 to 37 percent over Trible in a Senate matchup.

Trible, a conservative and a staunch defender of the Reagan administration's economic policies, has spent much of his Senate career in the shadow of the state's senior senator, Republican John W. Warner. As a junior senator, Trible has spent much of his time pursuing local issues, such as protecting the textile industry, and Roanoke River flood control.

Trible had hoped to use this summer's nationally televised Iran-contra hearings as a showcase for his talents, and initially received favorable press for his tough questioning of Richard V. Secord, a retired Air Force major general, and criticism of the White House for allowing the situation to get out of hand.

However, he infuriated many GOP activists who thought he had gone too far, and he later angered Democrats who said he "wimped out" in his questioning of Marine Lt. Col. Oliver L. North.

Nonetheless, in a poll conducted shortly after the hearings, 59 percent of the respondents gave Trible high marks for his performance during the hearings.

Although it will be weeks before the fallout from the decision is known, some observers speculated late yesterday that one or more of Virginia's five Republican House members might consider a Senate bid, including Rep. Thomas J. Bliley Jr. of Richmond and Rep. Stan Parris of Fairfax County. Parris has also given thought to entering the governor's race.

"I think you can safely say there are dozens of people out there -- on our side of the aisle -- who are suddenly thinking of running for the U.S. Senate," said Haner, the state GOP spokesman.

Trible, who was born in Baltimore and grew up in an affluent suburb of Scranton, Pa., has been a golden boy of Virginia Republican politics. His studied manner and calculated political moves frequently enraged Democrats, who called him "Plastic Paul."

Early in his career, he was a law clerk for a U.S. District Court judge in Alexandria. Later, he spent more than a year as a prosecutor in the U.S. attorney's office there.

In the summer of 1973, while President Nixon fought off subpoenas for his White House tape recordings, Trible was detailed to assist in the president's defense. He quit after three months.

In 1976, he was elected to the first of three terms as a House member from Virginia's 1st District, located in the Tidewater area. He went around the state for a year before 1982, anticipating that Sen. Harry F. Byrd Jr. might retire. When that happened, Trible was ready. That November he defeated Democratic lieutenant governor Richard Davis for Byrd's seat.

Staff writers Donald P. Baker, Ed Bruske and R.H. Melton contributed to this report.