Virginia lawmakers, quick to support tough stands on issues such as mandatory seat belt use, marital rape and drunken driving during this session of the General Assembly, have defied their conservative cast.

Yet, what may be best remembered about the session that ends Saturday is the legislators' use of pre-Watergate codes of ethics in an attempt to write themselves out of their own conflict-of-interest law -- a move that has brought a storm of public criticism that has obscured much of their other work. "The legislative biorhythm is out of sync this year," said veteran lobbyist Walter Marston, summing up the slightly out-of-kilter atmosphere that some say is a sign of transition in this capital city that once shunned change.

"There's a momentum toward moderation except in one area," the ethics issue, said Sen. Clive L. DuVal II (D-Fairfax).

Lawmakers suggest that several factors are at work in that trend toward moderation in a legislature that has often been called more conservative than its constituents. Two of those factors are the ripple effect of the political successes of moderate-conservatives after four years under Gov. Charles S. Robb, and the increasing urban-suburban shift in the legislature after decades of domination by rural interests.

At the same time, they say there is a kind of emotional exhaustion brought on by such loaded issues as a state lottery and limiting the abortion rights of minors, both of which were quickly defeated this year, and by the acceptance of some issues whose time simply had come. "It's sort of like the maturity of fruit on the vine," said Sen. Wiley F. Mitchell Jr. (R-Alexandria). "There seem to be a lot of things coming together at one time," he said, referring to several public safety issues that have passed or are close to passing this session.

"The failure of the extremes of both parties . . . has had an impact," said Mitchell, a moderate once shunned by the GOP hierarchy but now a member of the state executive committee. Mitchell said the Democratic Party has moved away from its liberal wing while Republicans are reassessing the role the far right will play in their party.

Sen. Frank W. Nolen (D-Augusta) admitted his surprise at the strong support for seat belt legislation and other issues that got nowhere in the past. "Where is it coming from?" Nolan wondered aloud, adding that the legislature is interfering with the people's "right to be foolish." Nolan said he has heard little support from his rural area for many of these issues except the drunken driving legislation.

DuVal, chairman of the Northern Virginia legislative caucus, said some of the change in atmosphere stems simply from the activism of Robb and his successor, Democrat Gerald L. Baliles.

"It still depends a lot on the character and desire of the governor," said DuVal. He recalled the activist first term of Gov. Mills E. Godwin from 1965 to 1969 but the relatively lackluster, managerial style of Godwin in his second term in the early '70s, which was similar to the term of GOP Gov. John N. Dalton, who preceded Robb.

In contrast, the assembly responded to what many considered Robb's dramatic plans to improve education and is going along with Baliles' effort to do the same for transportation.

Meanwhile, the internal machinery of the 140-member assembly is on the verge of major changes, probably after the 1987 elections, in which all 140 seats will be at stake.

The two current leaders of the legislature -- House Speaker A.L. Philpott, 67, from rural Henry County, and Sen. Edward E. Willey, 75, of Richmond, both Democrats -- are openly referred to as "dinosaurs" by Senate Clerk J.T. Shropshire, who is close to both men. Shropshire said that when they leave the assembly, the two will take with them an era of domination not likely to be seen again.

In the House, Del. Dorothy S. McDiarmid (D-Fairfax), in her first term as chairman of the Budget Committee, already has opened up consideration of the $18.5 billion biennial budget to more public scrutiny -- a move that irritates Willey, who controls the budget in the Senate.

In addition, the Senate and House increasingly are hiring professional staff members, many of whom work year-round researching issues. Lawmakers once were more dependent on the governor's office or the still-strong special interest lobbyists for such functions.

Many of the legislators still come from isolated areas where small newspapers and radio stations make little effort to cover them except for printing press releases.

Increasingly, however, news organizations throughout the state are paying more attention to the legislature, which Del. Theodore V. Morrison Jr. (D-Newport News) called "the biggest fishbowl of all" in state politics.

The divisive ethics issue has stirred up the "fishbowl" more than any other question this session. Even the normally reserved editorial page of the Richmond Times-Dispatch likened the assault on the ethics law to "roosters on a June bug" in one of several biting editorials on the issue.

Del. Clinton Miller (R-Shenandoah), a lawyer who argues that legislators should police themselves, has gone the furthest in complaining about the media's extensive ethics coverage, much of which has been negative.

Miller, who is from a rural area where he receives little critical attention in the media, has delivered two House speeches in which he disapproved of news coverage. In one he compared reporters to "a lynch mob" and claimed they had conspired to have a bad story written about him.

But Miller's attitude may be moderating, just as some say the assembly is. The day after his first speech criticizing the media, he was sitting in the House chamber, joking and laughing with reporters in the press section.