Transplacental and neonatal induction of mammary tumors (MT's) with ethylnitrosourea (ENU) was studied in Sprague-Dawley rats. A low transplacental ENU dose (10 mg/kg) did not increase the number of MT's or shorten their latency period. High ENU doses (30 mg/kg neonatal, 60 mg/kg transplacental, or 120 mg/kg transplacental) when corrected for differences in life span caused a significant shortening of the tumor induction period and an overall increase in the tumor incidence. With high ENU doses, the MT's were frequently multiple in the same animal and were more often malignant. Tumors developed mostly in females; only a few were observed in males. It is concluded that with a sufficient dose of the carcinogen in susceptible animals, transplacental and neonatal ENU mammary carcinogenesis takes place. The experiment was originally designed to evaluate ENU-induced neurogenic tumors; the results on MT's were obtained incidentally.