The effects of intravenous infusions of synthetic gastrin-releasing peptide (GRP; 5 pmol/kg X min) have been investigated in 3- to 6-week-old conscious calves receiving continuous intravenous infusions of either glucose or amino acids or both at a dose of 0.03 mmol/kg X min and the results compared with the effects of the same dose of the peptide in control calves. Pre-treatment with amino acids alone caused a statistically significant fall in mean plasma glucose concentration, which was associated with a significant rise in mean pancreatic glucagon concentration. Additional infusion of glucose prevented this rise in plasma glucose concentration and resulted in a delayed, but very substantial rise in mean plasma insulin concentration. Pre-treatment with amino acids alone substantially and significantly increased the rise in mean plasma insulin that occurred in response to GRP. The rise in mean plasma glucagon concentration in response to GRP that occurred in the control group, the group pre-treated with amino acids alone and the group given both glucose and amino acids, was virtually eliminated in the group pre-treated with glucose alone. The normal rise in plasma pancreatic polypeptide concentration in response to GRP was invariably abolished in the presence of amino acids. No significant change in either mean neurotensin-like or gastric-inhibitory-peptide-like immunoreactivity was observed in response to GRP in any of these groups. The results are discussed in relation to possible physiological functions that GRP may subserve.