The average scores of Prince George's County students who took standardized achievement tests in reading and math declined slightly reading and math declined slightly from 1977 to 1978, as did average general learning ability measured among most county students tested.

That is the finding in a report of the Maryland Accountability Assessment Program released yesterday. The tests were administered to third, fifth and seveth graders throughout the state last spring.

Prince George's students have traditionally has averages at or slightly below the statewide average on achievement, but the latest average scores generally fell farther behind the state averages in reading comprehension and math.

At the same time, county fifth-and seventh-graders generally gained on the statewide averages in language skills, the report showed.

Average scores on the cognitive abilities test, which measures aptitude and the report said is similar in intent and score meaning to an I.Q. test, deelined in grades three, five and seven last year after having increased in all three grades the previous year. Aptitude scores for ninth graders, who were also given this test, increased slightly after a large drop in 1976.

Victor Rice, the schools' supervisor for evaluation and research. concluded in the report that "in general, statewide average aptitude scores are slightly higher than those" for Prince George's. In light of that, he said, Prince George's average achievement scores "compare favorably with the state's average scores."

Rice noted that the test results showed that reading at grade five and mathematics at grade seven need improvement.

School officials said they were not discouraged by the test results and noted that separate study showed tht students' test scores rise with the length of time they spend in the school system.

According to the study, county fifth graders averaged a score of 5.2 on tests measuring grade level achievement if they had been in county schools for a year of less. Students in the system for four to five years or less. Students in the system for four to five years averaged a score of 5.6 on the same tests.

The only categories where average achievement scores improved were fifth and seventh grade language skills. The only category in which the Prince George's students matched the statewide average was in seventh grade language skill.