Midway through its most frenzied session in recent years, the Virginia General Assembly has killed measures calling for a state lottery, uranium mining and a ban on nonreturnable bottles and cans.

It has sidestepped prohibitions on "happy hours" and has once again turned down appeals from the electric utilities for a coal slurry pipeline.

While many emotional and much-debated issues already have been sent to the legislative graveyard, hundreds more await showdowns as the House and Senate trade bills this week.

The 100-member House of Delegates, faced with a calendar that kept lawmakers working until 2:30 a.m. today, has approved some of the most controversial issues of the session including a bill requiring most motorists to wear seat belts.

The House had so many bills on its calendar Sunday that they couldn't all be crammed into the binders on the legislators' desks. Even the House's teen-age pages were sent home while debate continued past midnight.

"We do have child labor laws in this state," muttered Del. C. Richard Cranwell (D-Roanoke), trying to find someone to make a soft-drink run, a task given the young pages.

Transportation issues have claimed much of the legislature's attention this year. The House-approved seat belt bill would make Virginia only the fourth state in the nation to require persons in the front seat of vehicles to use seat belts.

Proponents of a sweeping new plan to change the way the state hands out road money won a major victory today in a parliamentary battle that sent the plan to the Senate Finance Committee, which is considered friendly toward the bill. The bill could have ended up in the Senate transportation committee, which is more hostile to the proposal, according to many senators.

"We still expect a tough battle in the full Senate," said Sen. Joseph V. Gartlan (D-Fairfax).

The measure would pour more state road money into populous areas of Virginia, including the northern suburbs, at the expense of many rural areas. It has been one of the most heavily lobbied issues of the session, with city and suburban lawmakers attempting to convert crucial rural colleagues to their side in the fight.

Northern Virginia lawmakers, some of the most fervent supporters of the highway measure, have shut the press and public out of their weekly meetings to plot strategy.

"We're talking about whose arm we can twist and whose arm we shouldn't twist," said Sen. Clive L. DuVal II (D-Fairfax), chairman of the region's legislative delegation.

The House has approved another transportation bill that legislators say could affect the Washington suburbs, where frequent tractor-trailer accidents and mishaps on the Capital Beltway have become controversial. The House decided to allow longer trucks on the roads; lawmakers approved a 60-foot limit, up from the current 55-foot limit.

Both houses have passed their own versions of a law that would raise the drinking age to 21. Without the law, the state could lose millions of dollars in federal highway money.

Legislators are willing to spend more money on the state's troubled prisons, but less money than Gov. Charles S. Robb sought for guard salaries and mental health specialists for inmates.

They have tackled the issue of child abuse, approving more than a dozen measures on the issue. Those passed would allow videotaping of children's testimony in sex abuse cases, checks of the criminal records of persons hired by child care facilities and greater protections for children who accuse relatives of sexual abuse.

There have been a number of efforts to revise Virginia's voter registation laws. Several of them have been killed, but others calling for less sweeping changes remain alive.

The stage also has been set for the annual battle of the budget: the House and Senate each have presented their versions of the state's 1985-86 money bill. While the House would funnel budget surpluses into education, the Senate has proposed saving the money for the problems some senators say the state will face after the Reagan administration's planned cutbacks in state aid.