We have investigated the distribution, biotranformation, and excretion of streptozotocin and its 14C- and 3H-labeled metabolities in 15 patients with advanced cancer. Streptozotocin was detected in the plasma during the first 3 hours after administration while radioactive products were present for longer than 24 hours. No unchanged streptozotocin was detected in the cerebrospinal fluid (CSF); however, 14C-labeled metabolites were detected in the 2-hour CSF sample in a concentration equivalent to the 2-hour plasma level. H activity is the CSF was not detected at this time period. Radioactivity measured in biopsied tissues indicated that streptozotocin labeled with 14C and 3H or its metabolites penetrated tumor tissue. 14C tissue levels were found to approximate plasma levels; however, 3H levels were found to be greater than the corresponding plasma levels. Fifteen percent of the total dose of streptozotocin administered was recovered in the urine. 3H-labeled metabolites were recovered in excess of 60% in the urine, and approximately 30% of the 14C-labeled metabolites were recovered in the urine during a similar interval. Less than 1% of the administered 14C and 3H was recovered in the feces. 14C-labeled CO2 was also recovered, although quantitative recovery was not attained. At least three major metabolites of streptozotocin were detected in the urine by radiochromatography. Two metabolites contained only 3H and one metabolite contained both 14C and 3H in the same ratio as administered.