This study was designed to gain insight into the action of LHRH on the control of cellular proliferation in the anterior pituitary. The fraction of cells labelled with bromodeoxyuridine (S-phase cells) was studied in cytospin preparations of anterior pituitary cells taken from control male and female rats and from rats treated with daily doses of 2 micrograms/100 g body weight of LHRH (7 days), with doses of 40 micrograms of LHRH given on alternate days for 14 days (7 doses) or, finally, treated with daily doses of 50 ng of busereline acetate (14 days). Treatment with LHRH for 14 days resulted in a significant increase in the fraction of S-phase cells. However, neither the blockade of gonadotrophin secretion with busereline acetate nor its stimulation with LHRH for seven days resulted in a significant change in the proliferative activity of anterior pituitary cells. This action was independent of sex. No significant changes were seen in the proportions of pituitary gonadotrops of the different study groups. Regardless of the treatment-group very few cells doubly-immunostained for BrdU and LH were found. It is concluded that LHRH may stimulate cellular proliferation in the anterior pituitary, but further studies are needed to define which cells are involved in this action.