Pharmacokinetic-pharmacodynamic relationships in cytotoxic cancer chemotherapy are reviewed. In all classes of established cytotoxic drugs there is now good evidence that pharmacokinetics are a major determinant of drug toxicity and, in some instances, activity. Attempts are now being made to exploit these relationships to improve the therapeutic utility of established cytotoxic drugs and initial results are promising. In addition, there is evidence that the early clinical evaluation of new drugs may benefit from the application of pharmacokinetic information.