The oncogenic potential of adriamycin was studied in both sexes of Sprague-Dawley rats. Single i.v. injection of 10 or 2 mg and repeated injections of 2 mg of adriamycin/kg body wt within 2 weeks were performed on rats, 50 days old at the commencement. All surviving animals were killed at 52 weeks of the experiment. Multiple mammary tumors, mostly fibroadenomas, were observed in 30 and 41% of the females given single low (2 mg/kg) or repeated doses (5 X 2 mg/kg) respectively. These incidences were significantly higher than those for the corresponding control group (8%). Two male rats in the low dose group also developed mammary tumors. Both sexes receiving the single high dose (10 mg/kg) injection demonstrated early mortality due to marked toxicity. The mortality of five repeated-treatment group was lower than a single high-dose group. Renal cell tumors were evident in five rats of the single, low dose groups and dysplastic foci of renal tubular epithelium occurred in the groups given a single low or repeated doses of adriamycin. Thus the present experiment provides preliminary indication of carcinogenic potential of this chemotherapeutic agent for both kidney and mammary gland of Sprague-Dawley rats and further investigations are necessary to evaluate the carcinogenic risk of adriamycin treatment.