This study examines the effects of envisioning factors that can either facilitate or hinder performance on an upcoming activity. Prior to engaging in a cognitive task, subjects focused on factors that could enhance or impair their performance. They then indicated their perceived self-efficacy on the activity and performed the task. Dwelling on factors that could impair performance diminished judgments of self-efficacy. Group differences in subsequent task persistence paralleled the differences in self-efficacy judgments. Regression analyses provided evidence that judgments of self-efficacy mediated the behavioral effects. Subjects displayed nearly identical levels of performance motivation whether or not they recorded self-efficacy judgments. The results are interpreted in terms of the availability heuristic. Self-efficacy judgments generally may reflect a small set of highly available personal and situational factors; focusing on particular aspects of an activity heightens the availability of these features, biasing efficacy judgments, which, in turn, regulate subsequent behavior.