Abstract We examined the effects of the difficulty level of a task on two measures of mastery motivation: task persistence and task pleasure. Children of three age groups (12, 24, and 36 months) were given six puzzles of varying difficulty levels to complete. The results reveal that task persistence varied with the difficulty of the task. Infants and toddlers showed greater persistence at moderately challenging tasks as compared to difficult tasks. There was no effect of difficulty level upon degree of task pleasure; however, a significant increase in task pleasure did occur between 24 and 36 months. We hypothesized that the transition from sensorimotor to preoperational intelligence allows the infant to gain greater pleasure from the perception of his or her own effectance in acting upon the environment. Correlations between persistence and cognitive measures decreased with age, suggesting that motivation and cognition may become less interrelated with development. Implications for cognitive-motivational assessment are also discussed.