Involution of the enlarged pancreas was studied in rats changed from a diet of raw soya flour (RSF) to a normal diet of cubes. After feeding RSF for 4 or 12 wk pancreatic weight, DNA, RNA, and protein were all significantly greater than in control animals fed cubes continuously. When these animals were changed to a cube diet, pancreatic DNA and RNA returned to control values within 48 h, while pancreatic weight and protein reverted to control values within 7 days of the change in diet. The morphological changes seen in the pancreas during involution indicate that cell death occurred by the process of apoptosis. Increased cell death during involution was seen as a rapid increase in the incidence of apoptotic bodies (AB) located in the cytoplasm of intact acinar cells and macrophages. These AB's contained condensed fragments of cytoplasm, nuclear, or a combination of these remnants, which were derived from the acinar cell. The increase in apoptosis after withdrawal of the RSF diet was probably in response to the withdrawal of the trophic influence, cholecystokinin.