Anterior pituitary cells were studied with immunocytochemical methods 1-6 months after gonadectomy in male and female rats. One month after surgery, stained gonadotropes contained small scattered cisternae of rough endoplasmic reticulum, while others showed larger cisternae. The signet ring cells were observed 3 months after gonadectomy, and their numbers increased markedly by 6 months. Counts of freshly dispersed stained cells showed that in intact rats, the only significant sex differences were seen in the percentages of FSH and PRL cells. The females had higher percentages of both cell types. One month after castration, there were 2- and 3-fold increases in the percentages of LH and FSH cells, respectively. In females, ovariectomy resulted in a 2-fold increase in the percentage of LH cells and a slight but not significant increase in that of FSH cells. Three months after surgery, the percentages of LH or FSH gonadotropes increased from 8-12% to about 30% in both sexes. In the 6-month group, the percentages of stained gonadotropes were decreased to about 20% in both males and females. Counts of other pituitary cell types showed that the percentage of PRL cells in females declined to levels similar to those in the male within 1 month after ovariectomy. There was also a significant increase in the percentage of GH cells 1-3 months after ovariectomy. In contrast, the percentages of GH-producing cells decreased in the castrated males 3-6 months after surgery. The other pituitary cell types (ACTH and TSH) did not show significant changes after gonadectomy in either sex. The average area of gonadotropes was increased gradually, reaching a maximum 3 months after gonadectomy, after which it was decreased in the 6-month group. After ovariectomy, serum levels of gonadotropins correlated well with changes in areas of gonadotropes, showing a continuous increase until the third month, followed by a decrease after 6 months. Serum LH levels increased and then decreased after castration following the same pattern as that of the average areas. However, serum FSH levels continued to increase gradually throughout the entire 6 months. These studies are the first to quantify changes in all six pituitary cell types after gonadectomy. The counts confirm qualitative data reported previously that show changes in GH and PRL cells after gonadectomy (including sex differences). We extend previous studies that show that the expansion in gonadotropic cell area is correlated with a rise in both serum LH and FSH.