In rats injected with pentagastrin (2-500 micrograms/kg subcutaneously) in saline, peak gastric acid responses occurred after 31 (30-min output) or 63 micrograms/kg (60-min output). Rats were fed or fasted for 48 h and injected every 8 h with 63, 250, or 1000 micrograms/kg pentagastrin. Fasting decreased body weight (30% versus fed), serum gastrin (90%), and weight and protein content of oxyntic and pyloric gland areas, pancreas, small intestine, and colon. Deoxyribonucleic acid content or [3H]thymidine incorporation was decreased in all organs except colon. The lowest dose of pentagastrin significantly increased [3H]thymidine incorporation in the oxyntic gland area of fed rats and the small intestine of fasted rats, although organ weight, protein, or deoxyribonucleic acid content did not increase. These data indicate that short-term fasting has dramatic effects on gastrointestinal organ growth in rats and that pentagastrin reverses some of these changes.