The amino acids glycine, L-serine, L-asparagine and L-glutamine at 5 mmol/l each markedly increased glucagon release from perifused fetal lamb pancreas tissue, whereas the branched-chain amino acids L-leucine and L-valine had no effect. In contrast, only L-leucine and L-valine had any effect on insulin release. With perifused pancreas tissue from newborn lambs (5-9 days of age) glycine, L-serine, L-asparagine, L-arginine and L-lysine caused a similar marked increase in glucagon secretion with glycine having the greatest effect. These stimulatory effects were attenuated little by addition of glucose (20 mmol/l). L-Leucine had little effect on glucagon release, but was the only amino acid tested which caused marked insulin release in the absence of glucagon. Continuous intravenous infusion of glutamine (3 mmol/h per kg estimated fetal weight) or glutamine and asparagine each at this rate for 2 h into chronically cannulated fetal sheep in utero significantly increased plasma glucagon (P less than 0.05) and insulin (P less than 0.01) concentrations, although the effect on glucagon was not great. The results show how a range of amino acids can influence glucagon and insulin release from the pancreas of fetal and newborn lambs suggesting that physiological changes in plasma amino acid concentrations may contribute to regulation of glucagon and insulin release in utero in this species.