Gary R. Myers, the young Republican lawyer from Alexandria who scored one of the biggest political upsets in Northern Virginia two years ago, will not seek reelection.

"At some point you simply burn out." the 35-year-old freshman state delegate said yesterday. "I'm tired."

Myers, who defeated Democratic House Majority Leader James M. Thomson in a bitter campaign over civil and womens' rights issues said his decision was based on personel reasons.

"I've decided that my family is more important than politics," he said yesterday."It's exhausting trying to be a father, a husband, a lawyer and hold a part-time political job which is really full-time."

Myers' formal announcement at a meeting of the Alexandra Republican Committee, ended months of speculation by area politicians over his future. Myers had made no secret of the strain on his family and law practice that his two years in the Virginia General Assembly caused. "It's tough," Myers said yesterday.

Myers was the novice in a three-way race against Thomson and De. Richard Hobson (D-Alexandria). Myers' finished first thanks to support he garnered from women and blacks who were angered by Thomson's oppositon to the Equal Rights Amendment and his segregationist past. At the time of his defeat Thomson was in line to become chairman of the House Appropriations Committee and his defeat shocked both Democrats and Republicans in the legislature.

"What I represented was change," Myers said yesterday. Although he denied any "disillusionment" with the present system, he took a parting shot at campaigning, which he called "a form of entertainment. . . a popularity contest."

"I love politics," he said, "but that's don't think it's ineffective," Myers said. "Prudent would be a better word.

Myers-father of two-said he would not rule out running for public office in the future. "But for now, I'm going to devote myself full time to getting back to the people who are closest to me."

Although the Virginia legislature is regarded as a part-time job, its 140 members have to spend between two and three months a year in Richmond. They are paid $5,470 in salary and a maximu, of "2,288 in expenses each year, a level that may Washington area legislator say is too low for the amount of effort involved. CAPTION: Picture, GARY R. MYERS . . . "you simply burn out"