Within any course, as a semester progresses some students reduce their effort and others try harder. Virtually every cognitive theory of motivation suggests that changes in ability perceptions partially determine these changes in effort. Researchers have also cited changes in students' valuing of the course and changes in extrinsic pressures as determinants of effort changes. Covariance structure modeling was used to test 4 alternative models concerning the determinants of effort in a sample of 167 junior high school and 155 senior high school students. Models specifying a direct effect of ability-perception change on effort change fit the data better than did models specifying only indirect effects or no effect of ability perceptions on effort. Ability-perception changes also directly affected students' valuing of the subject matter. The results emphasize the importance of helping students develop confidence in their abilities.