The concentration of estrogenic, androgenic, progestational, and adrenocortical steroid hormones in body fluids of mature intact Sprague-Dawley female rats was increased by s.c. implantation of 5 to 7 mg NaCl pellets containing 1 to 20% steroid 48 hr before administration p.o. of either 7,12-dimethylbenz(a) anthracene or procarbazine. The incidence of rats developing one or more mammary carcinomas in each treated group was compared to that ovserved in simultaneously treated groups receiving only the carcinogen, steroid, or no treatment whatsoever, with weekly observation of all rats until palpably growing tumors were biopsied and proven carcinomatous or until death occurred from other causes determined by autopsy. A total of 105 untreated or steroid-implanted rats followed to death (234 to 256 days median observation) developed no breast carcinomas. Rats fed either of the carcinogens developed initial evidence of breast carcinoma, after 136 to 156 days median observation, in 51 to 57% of 318 total treated rats. Nonbreast carcinomas and sarcomas developed in 5 to 10% of the carcinogen-treated rats.