To determine the effect of elevations of plasma lipids on growth hormone secretion in humans, paired insulin hypoglycemia tests and paired arginine infusion tests were performed on eight and six normal female volunteers respectively. On 1 of the 2 test days for each growth hormone stimulus, subjects were given 60 g corn oil (Lipomul) 3 hr before testing followed by intravenous heparin (5000 U) at the time of insulin or arginine administration. Lipomul plus heparin administration inhibited both insulin- and arginine-induced plasma HGH elevations with almost complete suppression of the response to arginine. The plasma HGH (human growth hormone) inhibition was associated with elevation in plasma triglycerides and inhibition of plasma FFA (free fatty acid) depression after insulin or arginine. Neither the hypoglycemic response to insulin nor the blood glucose and plasma immunoreactive-insulin responses to arginine were altered by Lipomul plus heparin administration. In four additional subjects in whom Lipomul was given without heparin, the elevated plasma triglyceride values were not associated with suppression of arginine-induced plasma HGH elevations. In the same four subjects, heparin administration without Lipomul neither suppressed arginine-induced plasma HGH elevations nor prevented the depression in plasma FFA after arginine as much as when Lipomul plus heparin had been given. These latter observations suggest that the elevation in plasma FFA was responsible for suppression of growth hormone secretion by Lipomul plus heparin. These studies indicate a possible role of plasma FFA in regulation of growth hormone secretion.