A new mitomycin derivative, 7-N-(p-hydroxyphenyl)-mitomycin C (KW 2083), was labeled with 14C at 3 and 5 position of p-hydroxyphenyl moiety. Its radioactive purity and specific radioactivity were 97.6% and 3.42 mu Ci/mg, respectively. The labeled drug was administered intravenously to male rats or pregnant rats. Radioactivity was rapidly cleared from the plasma and transferred to the peripheral tissues. However low and sustained level of radioactivity was maintained in the plasma, even 48 to 72 hours after administration. About 29% of the radioactivity was recovered from the urine and 58% from the feces by 72 hours after administration. In the bile, 45% of the dose was excreted until 48 hours after administration, and the enterohepatic circulation was observed. The tissue levels of 14C-KW 2083 given to male rats were highest in the kidney, liver and intestinal contents. In the brain, eyes and spinal cord the radioactivity was very low. Autoradiography with 14C-KW 2083 in male or pregnant rats demonstrated that radioactivity distributed in kidney, liver and intestinal contents. The transfer of radioactivity was very low in the fetuses of rats. Radioactivity was observed to some extent in the fetal membrane. Plasma concentrations of KW 2083 or mitomycin C in dogs were determined by means of bioactivity and could be best described by a two-compartment open model. The total clearance of KW 2083 was higher than mitomycin C and hence the biological half life of KW 2083 was shorter. The result may suggest less toxicity of KW 2083 to the host cells.