In order to determine plasma amino acid concentrations during a prolonged but moderate insulin-induced hypoglycaemia, six healthy volunteers received a constant subcutaneous insulin infusion (15 mU.m-2.min-1) over a 12 hour period. The plasma glucose concentrations decreased from 4.72 +/- 0.11 to 2.83 +/- 0.07 mM at 600 minutes and then remained stable over the last 120 minutes. Plasma counterregulatory hormones (glucagon, epinephrine, growth hormone and cortisol) increased significantly between 120 and 180 minutes. The plasma concentration of all the amino acids paralleled the decrease in plasma glucose. The branched chain amino acids decreased to a greater extent in the first part of the study (0-360 min) in comparison to the essential non-branched chain aminoacids (p < 0.01), then increased significantly with a peak at 600 minutes (p < 0.05 vs 360 min) despite stable hyperinsulinaemia. These results suggests that during prolonged but moderate hypoglycaemia the counterregulatory hormones are able to antagonize partially the effects of insulin on protein metabolism, analogous to their well-known anti-insulin effects on glucose and fatty acid metabolism.