Thirty-nine adult female Long Evans rats were decapitated and the heads stored at room temperature. The pituitaries were removed at intervals from 30 minutes to seven days, fixed, embedded and studied by histology, immunocytology and electron microscopy. Histologically, changes were noticeable after two hours postmortem. Immunoperoxidase staining showed postivity for growth hormone, prolactin, FSH, LH and TSH up to seven days after sacrifice, appearing even stronger in the advanced stages of autolysis. Fine structural alterations were evident at 30 minutes and more conspicuous later. Changes included dilation, partial degranulation and whorl formation of RER, swelling of Golgi complexes and mitochondria, chromatin clumping, lysis, rhexis and pyknosis of nuclei, cytosegresome formation and disruption of cell membranes. Secretory granules remained well preserved throughout, although some exhibited fusion or reduced electron density. Dilation of capillaries with accumulation of erythrocytes, platelets and fibrin fibers were prominent findings. The severity of changes varied considerably from cell to cell indicating that the rate of autolysis is not the same among different cell types and is possibly affected by the actual functional state of the cell. It appears that increased membrane permeability and disruption of plasmalemma represent important steps in the autolytic process.