Fairfax County high school seniors had the highest scores on the Scholastic Aptitude Test last spring in the Washington area, but District of Columbia students, who still have the area's lowest SAT scores, made the greatest gains.

It was the second year in a row that Fairfax topped Montgomery and Arlington counties on the widely used college entrance examination, which is designed to measure verbal and mathematical ability.

The new results, reported to local school systems by the College Board, show that in Fairfax the average score on the verbal part of the test rose by one point to 458, while it fell one point in mathematics to 506. Meanwhile, the combined scores fell one point in Montgomery and three points in Arlington, the two districts that had seesawed throughout the 1970s as the highest-scoring in the area.

In District of Columbia schools the average verbal SAT score rose by 11 points from a year earlier to 334, while the mathematics score climbed by four points to 363. Despite the gains, the scores were far below the nationwide averages of 426 in the verbal and 467 in the mathematics sections of the test. The College Board reported this week that the national scores rose slightly last spring for the first time in 19 years.

A perfect score on each half of the two-hour, multiple-choice examination is 800. The lowest is 200.

It was the third straight year that SAT scores have risen in Washington schools, although the previous gains had been small. A spokeswoman for D.C. school superintendent Floretta McKenzie called the results "very encouraging." She noted that they were in line with higher scores on standardized reading and math tests over the past few years in the city's elementary and junior high schools.

"Mrs. McKenzie has stressed preparing for the tests and test-taking skills," said Janis Cromer, an assistant to the superintendent. "Of course, we want to do even better, but I think this is part of a general improvement in the schools."

About 29 percent of D.C. seniors took the SAT last spring, a relatively small proportion in comparison with suburban school systems because fewer city youngsters go to competitive colleges that require the test. The share of each D.C. senior class taking the test has held steady over the past few years.

The D.C. test-takers remain heavily black, reflecting the makeup of the school system, but the proportion of D.C. public school students tested who are white has risen from 5.8 percent to 8.8 percent in the past two years, according to College Board figures.

In the suburbs, the only school system to show an overall gain in SAT scores was Prince George's County. Its average score in mathematics increased by three points to 449, while its verbal score remained unchanged at 403.

Even so, Prince George's was the only suburban school system to score below the national averages on the test. In the past 10 years its scores have dropped slightly more than other large school systems in the area, alough its drop over the decade was substantially less than the nationwide decline.

"We recognize we are still below the national norm," said Prince George's schools information director Brian Porter, "but any little bit counts as we try to bring our kids up."

In explaining the increase in SAT scores nationwide, College Board officials suggested that it probably reflects tougher courses and stiffer grading after years of declining standards. There also has been more specific preparation for the test in high schools and through commercial coaching courses, though figures on this trend are not available.

In Alexandria, where the SAT verbal score went up six points, James McClure, guidance director at the city-wide T.C. Williams High School, said that the school is distributing vocabulary lists and is teaching word analogies. Both are important parts of the verbal examination, which also covers reading comprehension.

James P. Akin, executive assistant to Alexandria superintendent Robert W. Peebles, said that a 10-point drop in math scores may reflect weaknesses in doing multistep mathematics problems, which also show up on other tests. The mathematics part of the SAT concentrates on problem-solving skills, using arithmetic reasoning, basic algebra and geometry.

Akin said that Alexandria school administrators will meet next week to discuss how test results in the SAT and other examinations, may pinpoint changes that should be made in the schools.

"We realize that we might have some problems in math," Akin said, "and we're going to make sure that all the things our students need are in the curriculum."