This is a report by the National Assessment of Educational Progress that contains selected results from the Third National Assessment of Science conducted in 76-77, with emphasis on relative performance changes in achievement from 1969 to 1977. The report describes the data base and methods of measuring changes of performance. It reports on the general patterns of change in science achievement across three age groups; the trend is downwards for the 9 and the 13 year olds, but to a lesser extent than in the first and second assessments, while achievement level of the 17 year olds continues to decline over the three assessments. Students in rural areas have shown consistent improvement in succeeding assessments. The report shows that the achievement level of whites is higher than that of blacks, and that of males is also higher than that of females in all three assessments. In the three appendices at the end numerous tables of results are supplied. (GA) Primary type of information provided by report: Results (Change) (Selective).