The University of Maryland is planning to raise its admissions standards by spelling out for the first time what course prospective students must take in high school.

The new rules, which would go into effect from 1984 to 1988, were given preliminary approval by a university regents committee despite fears that they might reduce the number of blacks admitted.

The proposed standards would require applicants to pass at least 12 college preparatory courses, out of 20 required for high school graduation, with the university itself deciding which courses are rigorous enough to qualify. They would also require incoming students to get at least a C average in high school and a combined score of 650, out of a possible 1,600 points, on the verbal and mathematics parts of the Scholastic Aptitude Test (SAT).

University president John S. Toll, who recommended the changes, characterized them as a "bold move" in the face of an expected one-third decline in the number of high school graduates in Maryland by the end of the decade because of a drop in the teen-age population.

But Toll said the higher admissions standards were necessary to upgrade the university's academic standing and to cut back its remedial programs.

Besides attracting more top quality students from Maryland itself, Toll said he hoped the university could raise the proportion of nonresidents enrolled from 15 percent to a maximum of 25 percent, a proposal contained in the new admissions policy.

Members of the board of regents said they expected the full board to approve the changes at its next meeting in January.

"We're trying to put pressure downward on the high schools so there will be a better educational system for all," said Peter F. O'Malley, a regent. "This is a vehicle to get a quality student body."

In the past the University of Maryland has required its applicants to take no specific high school courses, and depended on SAT scores and grades, in no matter what courses students presented, to decide whom to admit.

Its new higher standards are in line with a move last March by five of the state's public four-year colleges to impose specific course requirements. During the past few years many state universities, including those in California, Texas and Connecticut, have raised their admissions standards.

Under the new requirements for Maryland, applicants will have to take four years of English in high school, three years of social studies, two years of laboratory science and three years of mathematics, including geometry and two courses in algebra. Because the standards for these courses might vary in different school systems, university officials will work with local school administrators to designate which courses in particular school systems meet university requirements. Toll said this proposal already has been discussed with county school superintendents. He said they generally support it.

At today's meeting, Regent Clarence Mitchell, former director of the Washington office of the NAACP, said he feared that the new requirements, particularly the minimum SAT scores, would reduce the number of blacks admitted. Toll said the new proposal, like existing rules, will allow 15 percent of each freshman class to be admitted without meeting the minimum requirements. He said this was necessary "for success in our affirmative action programs."

Blacks now account for about 8 percent of the university's approximately 36,000 regular undergraduates. He said about half of them are admitted under exemptions from current requirements.