Groups of rats were injected with either saline or pentagastrin (0.5, 1 or 2 mg/kg/day) for 14 days. Pancreatic weight, DNA, RNA and protein contents as well as the ability of the pancreatic polyribosomes to synthesize protein in a cell-free system were investigated. In all three pentagastrin-treated groups the weight of the pancreas was found to be significantly higher (33-73%) than in the control. DNA, RNA and protein contents were significantly increased by 47, 98 and 85%, respectively, in the group that received the highest dose of the hormone. With respect to ribosomal protein synthesis, while the pentagastrin dose of 1 and 2 mg/kg/day resulted in 25 and 72% augmentation, respectively, the dose of 0.5 mg/kg had no influence on protein synthesis. It is concluded that chronic administration of pentagastrin (2 mg/kg/day for 14 days) causes hyperplasia and hypertrophy of the pancreas.