We describe the clinical pharmacology and metabolism of 5-iodo-2'-deoxyuridine (IdUrd) during and after a 12-hour infusion. The kinetics of IdUrd were linear between 250 and 1200 mg/m2. The plasma IdUrd concentration reached steady state in less than 1 hour. Total body clearance of IdUrd was 750 ml/min/m2 and the disappearance t1/2 at the end of the infusion was less than 5 minutes. The primary metabolite, 5-iodouracil (IUra), did not reach steady state during the infusion. At the end of the 1200 mg/m2 infusion, the maximum plasma IUra concentration was 100 mumol/L, or about 10 times the simultaneous IdUrd plasma concentration. During the infusion there was at least a fifty- to 100-fold increase in uracil and thymine plasma concentrations. After the infusion, IUra disappearance from plasma was nonlinear, with an apparent Michaelis constant of 30 mumol/L. Plasma uracil and thymine levels slowly decreased after the IdUrd infusion until IUra fell to less than 30 mumol/L. There was subsequently a parallel and more rapid decrease in the plasma concentrations of uracil and thymine. Uridine, 2'-deoxyuridine, and thymidine plasma levels did not change significantly as a result of IdUrd therapy. These changes in endogenous pyrimidine pools are consistent with competitive inhibition of dihydrouracil dehydrogenase by IUra. An in vitro human bone marrow assay was used to determine the relative toxicity of IdUrd and IUra. Although exposure to IUra was tenfold higher than that to IdUrd, IdUrd was at least 100 times more cytotoxic to marrow cells.