This study concerns the implications of Peter Gollwitzer's concept of implementation intentions for Icek Ajzen's theory of planned behavior. Attitude, subjective norm, perceived behavioral control, and intentions were assessed before an intervention that required subjects to make implementation intentions concerning when and where they would perform breast self-examination during the next month. Behavior was assessed by self-report 1 month later. Results supported Gollwitzer's contention that goal intentions that have been supplemented by implementation intentions concerning where and when the behavior is to be performed are more likely to be enacted. Evidence suggested that implementation intentions were effective because they provided a mechanism that facilitated the retrieval of intentions in memory. Implementation intentions also reduced the capacity of past behavior to predict future behavior, suggesting that implementation intentions mimic the effect of habit in human action. Implications for applications of models of attitude-behavior relations are outlined.