Scholastic Aptitude Test (SAT) scores for the nation's college-bound seniors declined again this year, continuing a 10-year trend and defying most predictions.

A million students, roughly two thirds of the high schoolers going directly to college, took the tests to measure their verbal and mathematical reasoning abilities, according to today's report from the nonprofit College Board testing organization.

The report, called "National College-Bound Seniors, 1979," showed that the average SAT verbal score dropped 2 points to 427 after holding steady for two years, and the the average mathematics score fell 1 point to 467. The highest score possible is 800. In 1968, the verbal score average was 466 and the mathematics average was 492.

The report did not attempt to assess the cause of the decline. Robert G. Cameron, program service officer of the College Board testing division, said the results were "disappointing," but that there are many causes.

"Schools cannot expect to reverse the trend quickly or single-handedly," he said.

A 1977 analysis blamed some of the decline on television viewing, national turbulence, a changing family structure and a wider range of background among college-bound students, as well as looser teaching and learning standards.

Average scores on the achievements tests, of which students usually choose three out of the 15 available, also declined, the survey showed from 538 in 1976 to 529 this year.

Although male students scored higher than women on both verbal and mathematics tests, the difference was largest on mathematics scores: 443 to 493. More females than males took the tests, for the fifth consecutive year, although the overall number taking the tests has been about the same since 1972.

The number of students who called themselves minority group members hit a record 17 percent this year, 8.9 percent of the total being black. The median family income is up 8 percent from last year, to $20,800, but 70 percent of all respondents said they planned to ask for some financial aid.

Parents' ability to contribute to college costs varied dramatically by race. Blacks and Puerto Ricans said their parents would be able to pay about $410 per year, compared with $1,570 for whites. Average per-student charges are expected to be $3,250 at public four-year institutions, according to the Department of Health, Education and Welfare.

More students chose business as a college major than any other field, displacing health and medicine for the first time. "This switch is primarily due to women, whose interest in business has more than doubled since 1973," the report said.

The latest survey is sure to fuel the controversy over testing nationwide, which has resulted in "truth-in-testing" legislation in New York and similar proposals elsewhere.

A recent Federal Trade Commission report said that SAT scores could be improved as much as 25 percent by some of the coaching techniques now being marketed, an assertion disputed by the College Board. The notion of test validity is under fire in some quarters, with critics charging that testing organizations' secrecy about the structure, methods and assumptions of the tests often leave students with no recourse if a mistaken score erroneously labels them for life.

Educators had been hoping for a reversal of the trend in falling results this year because of renewed emphasis on basic educational skills and on tightened academic standards. However, "the reasoning abilities which the SAT measures develop slowly and stubbornly over time, both in and out of school," Cameron said.

The report said an increasing number of students with lower grade-point averages were taking SAT tests and aspiring to college. The most able students, judging by their scores, plan to study mathematics, physical sciences, English literature and engineering, while the lowest chose trades, home economics and education.