Two experiments based upon Gollwitzer's (1993) concept of implementation intentions are described. In both experiments, attitudes, subjective norms, perceived behavioural control and intentions from Ajzen's (1991) theory of planned behaviour were used to measure participants' motivation prior to an intervention in which participants made implementation intentions specifying where and when they would take a vitamin C pill each day. Behaviours were assessed by self-report and pill count at both 10 days and 3 weeks in Experiment 1, and at 2 weeks and 5 weeks in Experiment 2. Results supported the view that participants who formed implementation intentions were less likely to miss taking a pill every day compared to controls. Evidence suggested that implementation intentions were effective because they allowed participants to pass control of behaviour to the environmental cues contained in the implementation intention. Implications of the study and some suggestions for future research are outlined. Copyright © 1999 John Wiley & Sons, Ltd.