Six cell types of the pars distalis were studied from perinatal to senile stage in the golden hamster in combination with morphometric analysis and immunocytochemistry. GH cells occupied only a small area at 15 days of gestation and increased remarkably after birth to occupy a significantly larger area in the young adult than that in the other stages. PRL cells first appeared at 3 days after birth and increased rapidly thereafter to be distributed throughout the pars distalis. They always occupied a larger area in females than in males from 3 weeks after birth onward. ACTH cells were distributed mainly in the peripheral region of the pars distalis. They were dominant at 15 days of gestation, but relatively decreased after birth. The gonadotrophic cells were divided into LH cells and FSH cells. About 70% of LH cells were also immunoreactive to anti-FSH serum. LH cells occupied a larger area than that of FSH cells in every stage examined. TSH cells occupied the second largest area at 15 days of gestation, but rapidly decreased at 3 weeks after birth to occupy the smallest area among the immunoreactive cell types throughout the life-span. The chronological changes in the percentages of all cell types constituting the pars distalis seemed to reflect the activity of these cell types at a given stage.