To study the effect of the ovary on sexual differentiation of somatotrophs and lactotrophs, the anterior pituitary glands of castrated adult male mice which had received an ovarian transplant during postnatal development were studied using a stereological morphometric technique with an electron microscope. In adult male mice which were castrated neonatally and given ovarian transplants at the age of puberty (NCT-males), the ovaries contained follicles and corpora lutea. The percentages (approximately 40) and numbers (approximately 2 x 10(5)) of lactotrophs were similar in normal dioestrous females and NCT-males, but were higher than the percentage (9.3) and number (4.6 x 10(4)) in normal males. Ovarian grafts in adult male mice which were simultaneously castrated and given an ovarian transplant just before puberty (PCT-males) contained numerous follicles of various sizes but no corpora lutea. The percentage (46.8) and number (3.9 x 10(5)) of lactotrophs were greater in these mice than in dioestrous females. The percentage of somatotrophs in NCT-males (34.7) was less than in normal males (52.6), but was similar to that in dioestrous female mice (37.4). The percentage of somatotrophs in PCT-males (27.4) was less than in normal male and dioestrous female mice. These data indicate that lactotrophs and somatotrophs differentiate to the female phenotype when a cyclically functional ovary is present after puberty.