About 284,000 voters in parts of Fairfax, Prince William, Arlington and Loudoun counties and the cities of Fairfax and Falls Church are eligible to cast ballots next Tuesday to select Democratic and Republican nominees for 11 state and local offices, but if past trends are a guide, less than 10 percent will turn out. The polls will open at 6 a.m. and close at 7 p.m.

In Virginia, voters do not register by party, so the primary is open to all who live in the affected districts. Although voters must pledge to support the winner of the party primary, they cannot be made to vote for the party's nominee in the Nov. 3 general election.

Seven Democratic and seven Republican primaries are scheduled across Northern Virginia. A total of 29 candidates are running for various county supervisor seats in magisterial districts in Fairfax, Prince William and Loudoun counties, and eight hopefuls are seeking their party's nomination to run for the Virginia General Assembly.

The following is a county-by-county listing of election summaries. Information on the candidates and maps of the districts start on this page.Fairfax County Democrats

Voters will choose party nominees in two supervisor districts, Dranesville and Springfield.Fairfax County Republicans

Voters will choose party nominees in the Dranesville supervisor district, two State Senate districts, the 32nd and the 36th, and two House of Delegate districts, the 37th and the 38th. The 32nd Senate district includes a small part of northern Arlington County and Falls Church; the 37th House district includes the City of Fairfax.Prince William County Democrats

Voters will select party nominees in four supervisor districts, Gainesville, Brentsville, Woodbridge and Dumfries.Prince William County Republicans

Voters will choose party nominees in two supervisor districts, Brentsville and Gainesville.Loudoun County Democrats

Voters will decide the party nominee for the Catoctin District seat on the county board.

The election season has been dominated by development and transportation issues, with challengers striking an almost unanimous theme, that officeholders have not sufficiently restrained the area's booming growth and have offered few solutions to massive traffic congestion.

Historically, officials say, only party regulars and political activists vote in Northern Virginia primaries. Because of traditionally low turnout, the results are difficult to predict, and the candidate who can turn out a reliable if small block of voters has a decided advantage.

If a "throw-the-bums-out" sentiment is growing among the Northern Virginia electorate, as some political strategists think, the first signs of it may appear in primaries in which three incumbents are being challenged by opponents in their own parties.

In the most closely watched race in Prince William County, incumbent Edwin C. King will face Floyd C. Bagley, a former delegate to the Virginia General Assembly, in the Democratic primary for the Dumfries seat on the County Board of Supervisors. Assuming no independent candidates file by the 7 p.m. Tuesday deadline, the winner will be assured of capturing the board seat.

King and Bagley are party stalwarts and sometime political allies. Bagley has complained about King's inaccessibility and what he says are votes in favor of too many rezoning and special exception proposals. King said he has shown a responsible attitude toward growth.

The most crowded field is in the Prince William County Gainesville District, where Republican incumbent G. Anthony Guiffre is being challenged by fellow Republicans John W. Dempewolf Jr., an engineer at the IBM facility in Manassas, and civic activist Elizabeth Betty Duley. Robert L. Cole, a real estate agent, and Claude J. Bradshaw, a county high school history teacher, face off in the Democratic primary.

There are also dual primaries in Fairfax County's Dranesville District, where two-term Republican incumbent Nancy K. Falck is being challenged by Lloyd L. Thoburn, administrator of the Fairfax Christian School. Many see the race as a grudge match between the religiously conservative Thoburn and Falck, who has voted against rezoning two parcels of land owned by Thoburn and other Thoburn family members.

Democrats in the Dranesville District will choose between Lilla D. Richards, regarded as a knowledgeable civic activist, and Franklin S. Holland Jr., a political newcomer who describes himself as a conservative Democrat.