Prince George's County school board member JoAnn Bell, a last-minute entry in the 6th District County Council race, defeated heavily favored Council Chairman Gerard T. McDonough in yesterday's Democratic primary, apparently setting the stage for a dramatic realignment of the council this fall.

McDonough, a leading power broker in the county's once dominant Democratic organization, trailed Bell slightly in returns throughout the night, and conceded shortly after midnight with votes from only one of the district's 14 precincts uncounted.

Reached at a Democratic Central Committee post-election gathering, McDonough told a reporter: "I have lost. I called JoAnn and conceded and offered her the use of my files. She said I was a doll."

McDonough said he conceded when the results of his home precinct came in. "I won it, but I didn't win it by 2 to 1. I had to really win heavily in my end of the district to offset her end of the district."

McDonough's brother John, a leading county Democratic strategist, attributed the loss to the recent redistricting of the county, which did away with at-large council memberships in favor of nine single-member districts. " Gerard has always kept a county-wide perspective and that apparently did him in. If you don't tailor yourself specifically to your district, it can cost you."

The new single-member 6th District lies in the center of the county and includes the largely middle-class, integrated communities of District Heights and Largo.

In a second surprise primary result yesterday, the Rev. Perry Smith, a black Baptist minister, was defeated for the Republican nomination in the 5th Congressional District by unknown William P. Guthrie, a 28-year-old doctoral candidate, by a margin of about 2 to 1. The race attracted wide attention after Smith was feted at the White House earlier this year when he switched his party affiliation to the GOP and vowed to bring 10,000 black county voters with him.

Meanwhile, in the hotly contested Republican race for the county executive nomination, county Park and Planning Commissioner Ann Shoch, won about 53 percent of the vote and defeated TRIM amendment coauthor William J. Goodman. The winner will face County Councilman Parris Glendening, who overwhelmingly defeated three little-known challengers in the Democratic race.

In two hard-fought County Council races, Councilwoman Sue V. Mills defeated fellow incumbent David Hartlove in the south-county 8th District by a margin of about 3 to 2, while Councilman Roy Dabney was badly beaten by former school board member A. James Golato in the 4th, which includes Bowie and Greenbelt.

In three key Democratic State Senate races, State Sen. Frank Komenda easily outdistanced Del. Charles Blumenthal in the 26th Legislative District, and Sen. Thomas P. O'Reilly won handily in the 22nd over his last-minute rival, Del. Robert S. Redding, former chairman of the county's delegation to the House of Delegates. In the 25th District, B.W. Mike Donovan, of District Heights, won renomination over four challengers.

Shoch, 42, and Goodman, 52, a C&P Telephone Co. manager, were vying to replace County Executive Lawrence J. Hogan, the only elected Republican in the county. Hogan passed up a chance for reelection to seek the U.S. Senate seat held by Democrat Paul Sarbanes.

Each candidate tried to persuade voters that he or she was better able to carry on Hogan's conservative policies, and each enjoyed the backing of a different group of Hogan suporters.

Both outlined similar positions on enhancing economic development and bringing high-technology industries to the county, and both opposed modification of the property tax revenue cap known as TRIM.

Over the course of a two-year campaign, Glendening, 40, a two-term council member and University of Maryland professor, quietly locked up all major endorsements and Democratic Party support in this heavily Democratic county. Glendening is a strong supporter of the TRIM modification effort, on which voters will decide in the November general election.

The vote in yesterday's council races will result in a drastic reshaping of the body, which passes local laws and taxes and approves all zoning decisions. The council will shrink from 11 members to nine, and for the first time since the county adopted a charter government 10 years ago, each will be elected from single-member districts. Because of redistricting and other factors, half of the present members are certain not to return.

In the 8th District, which includes Marlow Heights, Oxon Hill, Camp Springs and Temple Hills, Democratic incumbents Mills and Hartlove waged a heated campaign for the seat, trading last minute charges of dirty campaign tactics.

The race for the Democratic nomination in the 5th District was a test of whether incumbent Dabney, who is black, would beat back challenges from three well-known challengers in the 85 percent white district. Dabney, a banker, had the support of the Chamber of Commerce as well as the county's Democratic senators, but Golato had represented the insular Bowie community on the school board for 13 years.

Three wide-open Democratic Council races had no incumbents running. In the 3rd District, James Herl, 29, a County Council staff member, won a paper-thin victory over four rivals, including lawyer Thomas Hendershot, who played a critical role in drawing the redistricting plan. He will face unchalllenged Republican Joseph Drewer.

In the 2nd District, Del. Anthony Cicoria won the Democratic nomination by a wide margin over Hyattsville Mayor Thomas L. Bass. In the 7th District, which is predominantly black, Hilda Pemberton, a county government worker, defeated two challengers.