Whole fetal human pancreases of 12-22 weeks gestation, showed histological growth and differentiation in vitro over 3 weeks. At glucose concentrations of 1-4 g/l, there was no difference in insulin secretion into culture medium over 1 h. There was no stimulation of insulin release by D-glyceraldehyde, thus defective glucose-stimulated insulin release was probably not due to impairment of an early step in glycolysis. In the presence of 0.5 mmol/l dibutyryl cyclic AMP, insulin secretion was enhanced (0.188 +/- 0.030 versus 0.100 +/- 0.012 mU x mg tissue-1 x h-1, p less than 0.001) independently of glucose concentrations. It thus appears that impairment of glucose-stimulated insulin release was unlikely to be due to insufficient intracellular cyclic AMP. Insulin release increased in response to tolbutamide and theophylline. Insulin secretion was stimulated in the presence of a fivefold increase in amino acid concentration (0.118 +/- 0.018 versus 0.031 +/- 0.008 mU x mg tissue-1 x h-1, p less than 0.001). There was a fourfold increase in basal insulin secretion from islets previously grown in high concentration of amino acids compared with standard culture medium, (0.284 +/- 0.052 versus 0.067 +/- 0.011 mU x mg tissue-1 x h-1, p less than 0.001), emphasizing the important role of amino acids as substrates for B cell metabolism and development.