Pharmacokinetic studies are now playing an increasingly important part in the preclinical and clinical development of new anti-tumour agents. Preclinically, pharmacokinetic, metabolism and toxicological investigations are being integrated and used for the rational selection of new agents with a favourable pharmacological profile. More importantly, this approach should reduce the chance of developing a lead compound that may display idiosyncratic and unpredictable behaviour in patients. Pharmacokinetic studies are also invaluable in identifying potential problems due to species differences in drug handling and should help improve the quality, reliability and predictability of toxicokinetic investigations in experimental animals. Importantly, pharmacokinetic studies involving PGDE, MTSE and Bayesian approaches are increasingly being used to guide dose escalation in early clinical trials. It is likely that these methodologies will evolve further as experience with them develops, and this should lead to improvements in the conduct and scientific quality of phase I/II trials. It therefore behooves the clinician and clinical pharmacologist to pay special attention to the design and implementation of pharmacokinetic studies in early clinical trials.