This study describes changes in cognitive persistence, a key measure of mastery motivation, between the ages of 10 (grade 4) and 14 (grade 8). Prior research in the field of mastery motivation has focused mainly on early childhood. No longitudinal research findings have been published about age changes in mastery motivation during the school years. The sample of this longitudinal study consisted of 372 students in Hungary and was representative of the Hungarian school population regarding parents' level of education. Participants were in 25 school classes, each from a different town. The instrument was the cognitive persistence scale of the Hungarian version of the Dimensions of Mastery Questionnaire (DMQ). Grade point average (GPA), parents' level of education, the size of the class and of the town were analyzed as background variables. Cognitive persistence based on students' self-reports decreased substantially in 61% of the students, did not change in 33%, and increased in only 6% over the four school years. Changes in cognitive persistence correlated with changes in GPA. Significant differences were found among school classes in both the average level of cognitive persistence and in how much it changed. However, neither cognitive persistence nor changes in cognitive persistence correlated with parents' level of education or town size. Aspects of the school and classroom climate seem to substantially impact changes in students' cognitive persistence. In addition, this study considered educational applications and for further research.