Over 3000 female C3H/Hen-MTV+ mice continuously received graded dietary levels (0, 2.5, 5, 10, 20, 40, 80, 160, 320, and 640 ppb) of diethylstilbestrol (DES) beginning at weaning. Mice were scheduled to be killed after 3 or 26 wk of exposure and were palpated weekly and removed for histological evaluation when subcutaneous masses reached 1 cm diameter. Mammary tumors were more prevalent than in controls only at 320 and 640 ppb DES. However, palpable mammary tumors appeared significantly earlier than in controls in mice fed 80 ppb and above. Mice killed at 3 wk and later showed a dose-response for several nonneoplastic endpoints. At 3 wk, moderate to severe uterine glandular hyperplasia, lack of corpora lutea, and vaginal keratinization were more prevalent than in controls at 80 ppb; cervical adenosis was more prevalent than in controls at 160 ppb and above. Generally, the prevalence of other nonneoplastic responses such as uterine fibrosis, stromal mucoid changes, and bony trabecular proliferation were increased above control levels only later than 3 wk at 160 ppb and greater. This study demonstrated neoplastic and nonneoplastic responses to DES at and above 80 ppb, but gave no clear evidence of either type of response below this level. Conclusions are, (1) dietary levels of DES causing nonneoplastic effects also cause neoplastic effects when fed chronically, and (2) neoplastic levels of DES may be predicted from a 3 wk feeding study in C3H/HeN-MTV+ female mice based on nonneoplastic responses.