A single oral solution dose (40 mg/m2) of 14C-prednimustine was administered to each of four cancer patients. Plasma, urine, and feces were collected at appropriate times and analyzed for total radioactivity. Plasma samples were analyzed for prednimustine. Peak plasma levels of radioactivity (1-3 micrograms 14C-prednimustine equivalents) occurred at 1.5-3 h in three patients and at 5-6 h in one patient. No intact prednimustine was detected in the plasma; this means that if present, it would be at a concentration of 0.02 micrograms/ml or less and would account for less than 1% of the total drug-related material at the time of peak plasma levels. Solvent-extractable metabolites had a plasma half-life of about 8 h or less. By 24 h essentially all the plasma radioactivity appeared to be covalently bound, and it was eliminated slowly with an estimated terminal elimination half-life of about 10 days. Rapid urinary excretion occurred in the first 24 h, and 40%-60% of the dose was recovered in the urine in 72 h. Although prednimustine was well absorbed, the ester was subject to extensive presystemic metabolism and was not present in the systemic circulation after oral administration.