Michael Marx of Portland, Ore., came from behind to regain the national foil title with 10-7 victory over Atlanta's Mark Smith last night in the men's final of the National Fencing Championships at George Mason University.

Marx, who won the title in 1977 and 1979 and now trains full-time for the 1984 Olympics, coasted through his first two bouts in the final round of eight. He encountered little difficulty until he ran up against defending champion Smith, a graduate student at Georgia Tech.

Marx fell behind quickly and trailed, 4-1, before he adjusted to Smith's aggressive attack, scoring four straight touches.

After he fell behind again, 7-5, Marx scored five straight times to win.

"I didn't have any problems until I faced Mark," said the 23-year-old Marx. "His attack was just incredible . . . and I couldn't do anything against it. So when he attacked, I attacked, too. I didn't stop charging him until the last touch."

Don Driskill, 31, a D.C. police detective and a surprise finalist, was eliminated in his first bout in the final round by Greg Massialas of San Jose, Calif., 10-4.

The unheralded Driskill was pleased with that, though. His only aim had been to advance to the round of 24, but he entered the finals seeded sixth. Massialas touched him three times quickly to go ahead, but Driskill rallied to make it 5-3 and 6-4 before Massialas closed out the bout.

"It's tough not having these people to fence against regularly and then going up against them like this," Driskill said. "It's a big joke around work that I fence, but it won't be when I come in to work Wednesday," he said.

Marx, who was seeded fourth in the final round, but only 13th entering the round of 24 yesterday, handily defeated Nestor Rosario of New York, 10-2, and James Bonocorda of New York, 10-3, to gain the final.

Smith, who was a member of the 1980 Olympic team and 1981 World Championship team, advanced with a 10-7 victory over Massialas, who defeated him last month in the final of the Cherry Blossom tournament at George Mason.

The tournament continues through Saturday, with the preliminary rounds of women's foil beginning this morning at 8 and the final round of 24 scheduled for 4 p.m. Tuesday.

Peter Westbrook of New York, a five-time national saber champion and two-time member of the U.S. Olympic team, will defend his title in the final round of the saber competition at 5 o'clock today.