Northern Virginia voters will go to the polls Tuesday to select eight Republicans and 10 Democrats to represent their parties in the state's legislative and county elections in November.

The polls in sections of Fairfax, Loudoun and Prince William counties and Fairfax City will be open from 6 a.m. to 7 p.m. There will be 99 polling places in Fairfax, 29 in Loudoun, and 45 in Prince William.

Because Loudoun and Prince William have countywide contests, polls will be open in all the precincts there.

Fairfax does not have any countywide primaries and 63 of the county's 162 precincts will be closed because party nominees were selected without opposition or by the action of the county's political parties.

"All the primaries are highly contested," said Fairfax County Republican chairman Ben Partin. "We expect a good turnout. There are an exceptional number" of candidates vying for the Republican nomination for some of the seats, said Partin, citing, in particular, the legislative seats being vacated by retiring Democrat State Sen. Adelard L. (Abe) Brault and Republican Del. Warren E. Barry.

Three Republicans--Robert E. Murphy, John W. Russell, Joe West--are running for Brault's 34th District Senate seat in central Fairfax, and four Republicans--Jack Caussin, Robert K. Cunningham, Bruce L. Green and William P. Whalen--are seeking Barry's 42nd District House seat in southern Fairfax.

The Democrats also are fielding a large number of candidates for those races. Two Democrats--Emilie F. Miller and Daniel S. Alcorn--are going for Brault's seat, while three Democrats--Lester Fettig, Mark L. Glaser, Lawrence J. Pascal--want Barry's seat.

The result has been, among other things, "an unbelievable sign war," said Dottie Schick, Fairfax County Democratic Party chairman. "I think there have never been this many signs for a primary."

There is one additional Democratic primary for a Virginia General Assembly seat--the race between Philip L. Chabot Jr. and Louis K. Rothberg for the 44th District seat in the Mount Vernon area currently held by Republican Del. Frank Medico.

Republicans have two additional contests. A race between Lonnie B. Bridges and Daniel K. Moller for Democrat Charles L. Waddell's 33rd Senate District seat that represents portions of western Fairfax and Loudoun counties.

Former Delegate Gwendalyn F. Cody is running against Lynne E. Purvis for the House seat representing the 38th District, which includes the Annandale area and is currently represented by Democratic Del. Nora Squyres.

Republicans also have primary contests for the Fairfax County Board of Supervisors seats in Springfield, where incumbent Marie B. Travesky is being challenged by Elaine McConnell, and in Providence, where Stephen A. Armstrong and John D. Austin Jr. are facing off for the right to oppose incumbent Democrat James Scott.

In Prince William County, four Republicans, Ronald A. Butler, Celia Sue Coe, John W. Dempewolf Jr. and Guy A. Guiffre, are fighting for the nomination for the Gainesville seat on the County Board of Supervisors and two, Harry M. Hittle and Frank E. Bolton, are vying for the nomination for sheriff.

Democrats have primary races for the Fairfax supervisor's post in Springfield between Gerry Serody and Thomas E. Giska and for the Dulles District seat on the Loudoun County board between Ann B. Kavanagh and Darrell E. Rodgers Sr.

There are countywide Democratic primaries in Loudoun for revenue commissioner between Lee T. Keyes and John S. Witul and sheriff among James F. Boyd, J. Roger Etienne and John R. Isom.

Other Democratic primaries include the Prince William supervisors race in Neabsco District, between John D. Jenkins and Edward Rodriguez, and in Gainesville District, between John E. Bonfadini and Donald L. White; and the Prince William sheriff race among Wilson Carlin Garrison Jr., David C. Mabie, and Bernard E. Wilkinson. Neither Democrat Schick nor Republican Partin would venture any predictions about the primaries, although both were willing to speak to the Nov. 8 general election.

"I think we have a fine group of candidates and they will fare well against the opposition," said Schick.

"I think 1983 is a Republican year," said Partin. "Reagan is still expressing the concerns of the American people better than anyone of this century."