Chronic reserpine treatment of adult rats results in the accumulation of pancreatic enzymes and reduction of their discharge. These changes are reminiscent of those in cystic fibrosis. Since the majority of cystic fibrosis patients have their pancreatic dysfunction manifested in childhood, we studied chronic reserpine treatment in rat pups. Four-day-old rat pups were given reserpine (50 µg/kg intraperitoneally) or vehicle daily until sacrifice. The reserpine group showed significant decreases in body weights at 14 and 21 days of age. Pancreatic weights were also decreased but were of normal weight or increased when normalized against body weights. At 14 and 21 days of age, pancreatic concentrations of amylase, lipase, and trypsinogen showed no difference between reserpine and control pups. At both ages, pancreatic contents of all three enzymes were generally less in the treated pups, but were found to be similar when corrected for body weights. Hydrocortisone treatment of 14-day-old pups caused precocious accumulation of pancreatic enzymes in both reserpine and control groups.Intestinal contents of lipase, trypsin, and amylase were decreased in the reserpine pups at 14 days of age and reached a more significant level at 21 days of age; these data suggest a decrease in the secretion of pancreatic enzymes. Dispersed acini from 14-day-old pups showed a reduced capacity to release amylase as stimulated by carbachol or the octapeptide of cholecystokinin. The results suggested that chronic reserpine treatment of pups in the suckling period did not cause significant disturbance of the developmental accumulation of pancreatic enzymes. A definite inhibition of exocrine secretion was found with reserpine treatment. In terms of suppression of enzyme secretion, the results with reserpinized pups resemble the conditions found in cystic fibrosis.