The pharmacology of BCNU, an active antitumor agent in animal and man, was studied with the use of the C14‐labeled drug. Radioactivity was excreted slowly in man and monkeys and rapidly in mice. Urinary excretion accounted for the maior portion of the isotope although as much as ten per cent was excreted as CO,. The compound is rapidly degraded and promptly after administration no intact drug is demonstrable, although plasma levels of the isotope are prolonged by protein binding of a portion of the drug. Part of the drug may be recirculated in the bile and partially accounts for the prolonged plasma levels. The drug is stable in an acid milieu and well absorbed orally. Its high lipid solubility allows it to readily cross the blood‐brain barrier. The active moiety of this agent is still unknown. The contrast of the short biologic half‐life of intact BCNU to the delayed clinical toxicity and prolonged plasma levels of carbon‐14 is interesting.