Pharmacokinetic studies are invaluable in understanding the effects of cytotoxic drugs on the body and in defining the distribution of drug to various tumours at different sites in the body. In individual patients pharmacokinetic studies may be a more sensitive indicator for dose adjustment or for a change of therapy than liver function tests or increased creatinine levels. However, many more studies are required to define relevant indicators of impending toxicity or, with more difficulty, response. Pharmacokinetic studies have become an important aspect of chemotherapy and will play an increasing role. Efforts to increase the amount of drug available for tumour uptake by, for example, delayed release preparations, the administration of prodrugs, liposome entrapped drugs and antibody targetted drugs will rely heavily on pharmacokinetic studies. Now that sensitive specific methods of analysis such as HPLC and RIA are available, pharmacokinetic studies are likely to become increasingly used in both clinical and research studies. Because of the complex interactions of the tumour and the host, it will be necessary for such studies to be interpreted alongside studies of the biochemistry and cell kinetics of tumour cells.