The significant effect that scheduling has upon the severity and types of drug toxicity has been known for many years. Evidence is available demonstrating that the schedule chosen will substantially effect the relative distribution of drug to various target organs. It has been shown that a likely cause for this with doxorubicin is that the efficiency of the various enzyme complexes responsible for disposing of the drug can be affected by scheduling. We believe a similar process can explain the marked effect that scheduling has on the pattern of 5-fluorouracil toxicity and present both clinical and computer data to illustrate this.