The number of prolactin and growth hormone cells was estimated in the mouse anterior pituitary gland. During postnatal development in females, the number of prolactin cells showed a rectilinear increase from about 0.3 X 10(4) at 20 days of age to about 21 X 10(4) in adult female mice, while growth hormone cells gradually increased from about 7 X 10(4) on day 20 to about 20 X 10(4) on days 60-70, with a plateau level of about 14 X 10(4) cells around puberty. Prolactin cells were more abundant in adult female (about 21 X 10(4)) than in male (about 5 X 10(4)) mice, while there were fewer growth hormone cells in adult females (about 20 X 10(4)) than in males (about 25 X 10(4)). Ovariectomy before puberty induced the male pattern of pituitary development by inhibition of the increase in the number of prolactin cells and stimulation of that of growth hormone cells. In animals ovariectomized just before puberty (30-35 days of age) or 5 days after vaginal opening (40-45 days of age) the number of prolactin cells showed no further increase after the day of operation. On the other hand, when animals were ovariectomized at 10 days of age, the number of prolactin cells increased to the level found in 30-day-old normal control females. This implies that the increase in prolactin cells between 10 and 30 days of age is not dependent on the ovary, although after puberty the increase does depend on the presence of the ovary.