The Virginia General Assembly, the nation's oldest continuous legislative body, convenes here Wednesday with the 140 lawmakers expected to concentrate on carving both a $183 million state budget surplus a new legislative districts.

Republican Gov. John N. Dalton is expected to propose earmarking most of the surplus funds for salary increases for state employes, education programs, and to make up the deficit in the state's Medicaid budget. The conservative leadership of the Democratic-controlled legislature is likely to go along, but a substantial group of legislative mavericks plan to challege the governor and the assembly leaders by offering a variety of tax-cutting proposals.

Some, including House Finance Chairman Archibald Campbell, suspect the surplus could be twice as high as the $183 million figure Dalton has released. A number of lawmakers, led by Del. Warren Stambaugh (D-Arlington), want to use the money to gradually repeal the state's 4 percent sales tax on food, a perennial issue here. Others would prefer a direct rebate to taxpayers through state income tax reductions.

Reapportionment could prove another challenge to the assembly's leadership, which in the past has carved up legislative and congressional districts with an eye toward preserving the electoral advantage of both Democratic and Republican incumbents. This year, with 1980 census data showing sizable population growth in the GOP-dominated suburbs, some Democrats say they will push for a more aggressively partisan redistricting in an effort to prolong their 3-to-1 domination of the legislature.

Unlike last year's session when much of the focus was on state funding for Metrorail, the assembly will face few strictly Northern Virginia issues. But a number os social issues, including a proposal to raise the state beer and wine drinking age to at least 19 and a measure to ban sales of drug-related paraphernalia, could have a major impact on the Washington suburbs.

The assembly will consider a Fairfax County-backed plan to require smoke detectors in major apartment complexes and other public buildings, a proposal that could run into strong opposition from influential construction interests. Northern Virginia consumers would also be affected by a proposal by the powerful retailers' and bankers' lobbies to raise credit card annual interest charges from the present 18 percent to a maximum of 24 percent.

Many legislators believe 1981 could be the year when the state's rape laws are finally reformed to give victims additional protections in reporting and pressing prosecution of rape cases. In past years, the bill has been stalled by defense lawyers who dominate the House of Delegates committee that must approve the measure.

The only major environmental legislation likely to be considered this year is a proposal to protect the state's ecologically-fragil wetlands areas. Last year, the assembly passed a similar measure only to have it vetoed by the governor at the behest of the state Chamber of Commerce, which felt the bill would hurt developers. Proponents believe they have the votes to pass the bill again this year and, if necessary, to override a gubernatorial veto.