The ultrastructural changes of rat corticotrophs were studied with the protein-A-gold immunocytochemistry. The number of secretory granules per square micrometer of cytoplasm decreased after adrenalectomy and remained low. This change was mainly due to the decrease in the mature granules. The number of gold particles on the secretory granule was increased 3 or 7 days after adrenalectomy, reflecting the high immunoreactive adre’nocorticotro-pin content in these states as has been already reported. The number of coated pits or coated vesicles beneath the plasma membrane was not increased significantly, and exocytosis was only infrequently observed. Ultrastructural changes which suggest a secretion pathway other than the secretory granules were not observe