The rate of development of mammary tumors in (C3H X RIII)F1 hybrid mice ovariectomized at 2 months of age was similar to that of mice ovariectomized at 3 months. The increased rate of tumor development in many females ovariectomized at 4 or 5 months over that of females ovariectomized at 2 or 3 months indicates that the ovaries before their removal affected the subsequent development of mammary tumors. In the controls, the mean tumor age was much lower and the tumor incidence higher than in the ovariectomized females. Nodular hyperplasia of the adrenals was present in all tumor-bearing mice spayed at 2 or 3 months of age and in many of those spayed at 4 or 5 months. These findings, with the use of F1 hybrids of C3H female X RIII hermaphrodite, agree with the postulate that hyperplastic adrenals of spayed mice of the C3H strain secrete estrogenic substances capable of inducing mammary tumors.