Thiotepa has been used clinically for greater than 30 years but its pharmacokinetics remain poorly defined. We determined the plasma pharmacokinetics and urinary excretion of thiotepa and its metabolites in 21 patients with breast cancer who received 25 courses of iv bolus thiotepa (12 mg/m2) as part of combination chemotherapy. Plasma samples were obtained before injection: at 5, 10, 15, 30, 45, 60, 90, and 120 minutes; and, when possible, 180 and 240 minutes after injection. In eight courses, urine was collected as 4-hour aliquots for 24 hours after therapy. All samples were analyzed for thiotepa and tepa by gas-liquid chromatography. Urinary alkylating activity was assessed spectrophotometrically after reaction with 4-(p-nitrobenzyl)-pyridine. Plasma concentrations of thiotepa declined in a biexponential fashion with an alpha-half-life of 7.7 +/- 1.2 minutes and a beta-half-life of 125 +/- 21 minutes. Total-body clearance of thiotepa was 186 +/- 20 ml/minute/m2. The volume of the central compartment was calculated as 0.25 +/- 0.04 L/kg, and the steady-state volume of distribution was calculated as 0.70 +/- 0.11 L/kg. Tepa was detectable in plasma by 5 minutes after the injection of thiotepa. Tepa concentrations increased from 0.093 +/- 0.068 to 0.127 +/- 0.11 micrograms/ml over the 240-minute collection period. By 120 minutes, the concentration of tepa equaled that of thiotepa, and tepa persisted longer in the plasma than did thiotepa. During the first 24 hours after injection, urinary excretion of thiotepa, tepa, and alkylating activity accounted for 1.5%, 4.2%, and 23.5% of the administered dose, respectively. These results extend our laboratory's previous animal studies of thiotepa and argue for metabolism of thiotepa to tepa as a major mechanism of clearance of this compound. Further metabolism or breakdown of both compounds may explain the urinary excretion of alkylating materials other than parent compound and tepa.