SUMMARY: An early effect of castration on the cytology of the rabbit adenohypophysis was found to be a hyperplasia of the mucoid cells of the zona tuberalis occurring simultaneously with a degranulation and decrease in the numbers of such cells in the lateral regions of the pars distalis proper. A recovery from these changes was observed about 4 months after castration. There was then a moderate hyperplasia of the mucoid cells of the pars distalis proper and probably a depletion of mucoid cells in the zona tuberalis. The possible significance of these findings is briefly discussed.