Early ultrastructural and immunohistochemical changes caused by supramaximal secretory stimulation with caerulein were studied in the rat pancreas. The morphological basis for the earlier reported decrease of pancreatic juice secretion after supramaximal caerulein was the appearance of swollen and irregular zymogen-like material containing structures with short segments of budding bristle-coated membranes in the apical parts of acinar cells. Images of exocytosis of zymogen granules were only few. Later, marked vacuolization and signs of autophagocytosis are seen in the basal cytoplasm. Immunohistochemistry showed that the large zymogen containing structures were intensively labelled for trypsin at the early stages of the experiment (4-30 min). Later (1-2 h), the vacuoles were empty or contained occasional, small-labelled granules only. The pancreozymin-receptor antagonist proglumide as well as cycloleucine that inhibits protein synthesis by inhibiting the synthesis of S-adenosylmethionine, effectively prevented the caerulein induced acinar cell changes. The irregular zymogen containing structures with coated pits on their surface indicate disturbed zymogen granule formation leading to the accumulation of large lakes of zymogen material and finally to marked autophagocytosis in acinar cells. The effects of caerulein are receptor-mediated and depend on the process of methylation in the formation of zymogen granules.