In the stock used there was a 5 % incidence of mammary carcinoma in 9 mice after normal breeding and suckling of the young, tumors appearing when the animals were 18-24 mo. old. Two groups of [female] mice were bred earlier and oftener than customary and the litters were quickly removed after parturition. The mice in one group were first bred at 2-3 mo.; in the other, at 6-9 mo. In a 3rd group, various combinations were made of periods of suckling alternating with periods of non-suckling, thereby preventing proper drainage of the mammary gland and producing marked stasis in the ducts. Results indicated that in mice breeding rapidly from an early age and not suckling their young, the mammary glands were prevented from developing normally and more Jitters were necessary before carcinoma developed; in mice treated similarly but older when first bred, or in those in which suckling had once established large, well developed glands, fewer pregnancies followed by non-suckling of the young were necessary for appearance of carcinoma. Of the experimental animals 87% developed mammary carcinoma, contrasting sharply with the 5 % incidence in control stock.