Walt Whitman High School in Bethesda continues to lead the Washington area in producing semifinalists for National Merit Scholarships, but its preeminence in the annual competition has substantially declined.

Twenty-four students at Whitman have qualified as semifinalists this year, a number that is followed closely by 22 students at Langley High School in McLean, according to test results issued by the National Merit Scholarship Corp.

Two other high schools in Montgomery County, Wootton and Bethesda-Chevy Chase, and one in Prince George's, Eleanor Roosevelt, also have 15 or more semifinalists each.

For a full list of the area's national merit semifinalists, see today's Weekly section.

During most of the 1970s, Whitman regularly had more than 30 Merit semifinalists a year. It reached a peak of 35 semifinalists in 1975, almost triple the number from any other Washington-area high school and the second highest in the nation among approximately 18,000 schools whose students take the qualifying exam.

This year the number of semifinalists at Whitman ranks 19th among schools nationwide.

The Merit Scholarship Corp., a nonprofit group that has been conducting the scholarship competition since 1955, urges that the test results not be used to measure the quality of schools. The corporation warns that a variety of factors, including ability and family background, influence test results. But its lists of semifinalists have been widely cited to identify exceptionally bright students and show where they go to school.

"It's easy to abuse some of these numbers," said Denis P. Doyle, director of education policy studies at the American Enterprise Institute. "But these are the nation's smartest students. The fact that a high concentration of them goes to a relatively small number of schools probably indicates that a sound education does make a difference."

A compilation of Merit test results also shows that public schools in Fairfax and Montgomery counties and private schools in the District of Columbia continue to dominate the competition.

However, after slipping for several years, Arlington County had the highest proportion of students qualifying as semifinalists--2 percent of its current senior class--of any major school system in the area.

The 15,000 students selected as Merit semifinalists nationwide represent about .5 of 1 percent of the nation's high school seniors.

Only three students in Washington public schools, all of them at Wilson Senior High, qualified as semifinalists, compared to 49 students at D.C. private schools, even though public school enrollment is about 3 1/2 times greater than private schools'.

The Merit semifinalists were picked on the basis of a 100-minute multiple-choice test in English and mathematics, given last fall to about 1.1 million high school juniors throughout the country. The exam is similar to the Scholastic Aptitude Test (SAT), sponsored by the College Board.

This year Stuyvesant High School in New York City had the most semifinalists, 65, a position it has held for the past six years.

Besides Whitman and Langley, schools in the Washington area with 10 or more semifinalists each include four other Montgomery County public schools-- Wootton (16 semifinalists), Bethesda-Chevy Chase (15), Springbrook (13) and Rockville (12); two other Fairfax County schools-- W.T. Woodson (14) and Madison (10); one Prince George's public school, Eleanor Roosevelt (15); and one private school in Washington, Sidwell Friends (13).

John Keating, guidance director at Walt Whitman High, said the decline in the number of its Merit semifinalists parallels a drop in enrollment, caused by a decrease in the school-age population in its attendance area. The proportion of seniors qualifying on the test, about 5 percent, is as high as it has ever been, Keating said.

At Langley the number of semifinalists has increased substantially even though enrollment is down by about 10 percent over the past five years.

All of the area public schools with large numbers of Merit semifinalists draw students from upper-middle-income neighborhoods except Eleanor Roosevelt High in Greenbelt, which has an advanced program in science and mathematics that takes students by competitive admissions from about half of Prince George's County.

Doyle noted that across the country public schools with selective admissions do well in the Merit competition, along with private schools and some comprehensive public high schools in the suburbs. Nonselective public schools in big cities have almost disappeared from the Merit lists, Doyle said.

"I think there's a very strong preference for high quality, demanding schools among educated, middle- and upper-income parents," Doyle said. "If they can't get it in a public school, they either move away or buy it."

This year Washington's new selective public high school, Banneker Senior High, has its first class of seniors, but none of them qualified as a Merit semifinalist.

Even though all students in the Merit competition take the same test, the number of semifinalists for any state or the District of Columbia is determined in proportion to the size of its high school graduating class. Thus, the lowest score needed to qualify as a semifinalist differs from state to state. This year's cutoff scores include 204 (out of a possible 240 points) in D.C., 203 in Maryland, 200 in Virginia, 201 in New York, and 178--the nation's lowest--in Mississippi.

Because of the very strong showing of its private school students, the District has the highest qualifying score in the nation.

The 137 semifinalists in Fairfax County public schools account for 42 percent of all the semifinalists in Virginia. The 123 in Montgomery County make up 43 percent of those in Maryland. Both school systems have only about one-seventh of their states' high school graduates.

Next spring about 90 percent of the Merit semifinalists will be named finalists in a process that involves school recommendations and grades as well as test scores. About 5,300 will be awarded Merit scholarships for college.