Hyperprolactinemia and prolactinomas are among the abnormalities reported for women exposed prenatally to diethylstilbestrol (DES). To pursue this issue in an animal model replicating the other abnormalities of prenatal DES exposure, pituitary glands were studied in the offspring of CD-1 mice receiving an i.p. injection of 1 or 2 µg DES/g body weight during late pregnancy. Among 132 mice exposed prenatally to DES and then raised to terminal illness, there were 24 pituitary tumors compared to only 1 tumor among 64 controls. The tumors consisted predominantly of cells with an eccentric nucleus and cytoplasm characterized by an acidophilic core and basophilic rim. These cells were identified as lactotrophs on the basis of prolactin immunohistochemistry and by an expected variation in frequency relative to physiological states. Evaluation of ovaries from the same mice revealed a deficiency of corpora lutea and an elevated incidence of ovarian tumors. These findings are consistent with abnormal sex differentiation of the fetal hypothalamus being the cause of most adverse effects from prenatal DES exposure.