We have reported a high incidence and sex linkage of breast cancer in Wistar rats following gastric instillation of methylcholanthrene (J. Nat. Cancer Inst., 10:255, 1949, and 13:307, 1952). For this report, reproducibility of method and effects of estradiol and testosterone are evaluated in 103 male and 169 female rats which received methylcholanthrene 6 times a week throughout the experiments. The 169 female rats represent a pool of groups of 10 or more animals used as controls for experiments run during the past several years. Tumor incidence ranged from 73-100%, with a combined average of incidence 82%. The high incidence of pituitary tumors in both sexes treated with estradiol; the decrease in breast cancers in methylcholanthrene-treated females receiving testosterone or estradiol; and the incidence of breast cancers in both sexes receiving only estradiol permit speculation regarding the role of the pituitary gland in induction of breast cancer by methylcholanthrene in our experiments. The sharp drop in incidence of breast tumor in animals receiving methylcholanthrene and estradiol suggests involvement of a pituitary factor or factors in induction of breast tumors by methylcholanthrene; with the development of pituitary tumors in animals receiving estradiol, these factors may no longer be available. Estradiol-induced breast cancer in the rat may utilize a mechanism independent of the pituitary.