The effect of hydrocortisone (75 mg/kg) on antral, duodenal and pancreatic gastrin concentrations and on intestinal lactase, sucrase, maltase and alkaline phosphatase activities was investigated in suckling rats. Antral and pancreatic gastrin levels in normal 4- to 22-day-old rats were also determined. Hydrocortisone was injected daily to 7- and 10-day-old rats for 6 days. At the end of the experimental period the animals were 12 and 15 days old. Control groups were injected with saline. Hydrocortisone administration caused a profound induction in sucrase activity and markedly stimulated maltase and alkaline phosphatase activities in both age groups. After hydrocortisone administration 12-day-old rats showed a slight (28%) but significant stimulation in lactase activity, whereas in 15-day-old rats the enzyme activity was significantly decreased by 23%, compared to the respective saline control. Gastrin concentration in the antrum increased steadily between 4 and 22 days of age, whereas in the pancreas it decreased sharply from a relatively high level in 4-day-old rats to an essentially undetectable level in 22-day-old rats. Following hydrocortisone administration gastrin concentration in the antrum of 12- and 15-day-old rats was found to be significantly increased by 104 and 47%, respectively, but in the pancreas it decreased by 44 and 57%, when compared with the corresponding saline control. Hydrocortisone caused no apparent change in duodenal gastrin concentration in 12-day-old rats but produced a nonsignificant 35% increment in 15-day-old animals. The observed changes after hydrocortisone treatment are thought to be the result of an early maturation of the gastrointestinal mucosa and pancreas by the steroid.