A regulatory role of acute changes in plasma concentration of free fatty acids on glucagon secretion has been suggested. We have studied the effect of such changes on plasma levels of glucagon, insulin, and growth hormone in man. Basal plasma levels of immunoreactive glucagon (IRG) were only slightly raised in 11 healthy subjects when the mean concentration of free fatty acids (FFA) was depressed to levels as low as 0.3 15 f 0.043 (SEM) mM by infusion of nicotinic acid. Basal levels were increased modestly when the mean FFA level was elevated to 3.027 f 0.184 mM by infusion of a triglyceride emulsion (Intralipid) with heparin. The plasma IRG response to intravenous arginine was unaffected by high or low levels of plasma FFA. These findings contrasted with the effects upon plasma levels of immunoreactive insulin (IRI) and growth hormone (IGH). During elevation of F7A levels, the mean basal level of plasma IRI increased by lOO%, and the IRI response to arginine increased by 50%. Concomitantly, basal IGH levels and the plasma IGH response to arginine were suppressed markedly by elevation of FFA levels. The results of these studies do not offer support for a significant role of variation in plasma level of FFA as a regulator of acute changes in plasma IRG in man. An influence of changing levels of FFA on insulin secretion was found, and an effect on levels of growth hormone was confirmed. M ETABOLIC SUBSTRATES such as glucose, amino acids, and free fatty acids (FFA) have important regulatory effects on a variety of hormones that influence the transfer of these substrates to or from energy stores. Rising plasma levels of glucose stimulate secretion of insulin and suppress secretion of glucagon and growth hormone. Opposite effects occur when plasma glucose levels are falling or hypoglycemia is induced. Rising levels of certain amino