Increasing numbers of persons with heart, lung, kidney, musculoskeletal, and behavioral disorders who take medically prescribed drugs are choosing an active lifestyle. However, we know little about the effects of exercise on drug metabolism or how drugs might affect exercise performance. In this paper some basic concepts underlying pharmacokinetics (the study of determinants of drug concentration) and of pharmacodynamics (the study of the biological effects of drugs) will be considered. Although there is relatively little data related to exercise, an understanding of these pharmacological principles can be widely applied.