The mechanism of acinar cell loss occurring during ethionine-induced atrophy of the pancreas was investigated. Rats were given a standard diet, a protein-depletion diet (PDD), or a PDD with low- (0.04 g/kg body wt; LDE) or high- (0.4 g/kg; HDE) dose ethionine administered intraperitoneally daily for 10 days. Changes were most extensive in the animals given a PDD and HDE: After 10 days, pancreatic weight was reduced by 72%, and most of the acinar cells had disappeared. Prior to their deletion, these cells showed cytoplasmic vacuolation and enhanced autophagy. The main mechanism involved in their deletion was apoptosis, the apoptotic bodies being phagocytosed and degraded by adjacent acinar cells and intraepithelial macrophages. In contrast, necrosis of acinar cells was rare. Interstitial inflammation and apoptosis of capillary endothelial cells were also observed. In animals given a PDD and LDE, enhanced apoptosis occurred later and was more limited in extent, and additional manifestations of cell injury were not evident. As in other circumstances where glandular atrophy is effected by apoptosis, the basic tissue architecture was preserved, thus explaining the known capacity for the pancreas to regenerate after ethionine is discontinued.