SUMMARY: Pieces of pancreas from human foetuses of 14–24 weeks gestational age and weighing between 50 and 625 g were incubated in vitro. Insulin release was studied under control conditions and in media supplemented with glucose (3 mg/ml), leucine (5 mmol/1) or arginine (5 mmol/1). Glucose never caused a significant rise in insulin release from the pancreas. The failure of mannoheptulose (3 mg/ml) and 2-deoxyglucose (3 mg/ml) to suppress basal insulin release in a glucose-free medium indicated that basal insulin release was not governed by the liberation of glucose from glycogen in the exocrine pancreas. Arginine stimulated insulin release in four experiments using pancreas from foetuses weighing more than 200 g, but failed to cause insulin release from the pancreas of foetuses weighing less than 200 g in three experiments. Leucine consistently stimulated insulin release from the pancreas of foetuses of less than 200 g body weight but was only variably effective in causing insulin release from pancreas of foetuses weighing more than 200 g. The experimental results illustrate the development of different mechanisms for the release of insulin from the human foetal β cell.