Following the intravenous injection of fat (Intralipid) with heparin into five unanesthetized adult male baboons, the serum FFA level rose to a mean (±SE) of 11.30 ± 3.58 meq/liter at 5 min and the serum GH concentration fell rapidly from a mean base-line value of 8.1 ± 2.0 ng/ml to 3.4 ± 1.4 ng/ml 20 min after injection (T 1 2 = 19 min. This fall in serum GH was more consistent and of greater magnitude than that following the injection of either fat or heparin alone. A late rise in GH was not observed through 2 hr. The depression of serum GH was not explicable on the basis of hyperglycemia. The rapid fall in the serum GH concentration after the injection of fat with heparin, with a serum halftime approximating the disappearance rate of GH in man, the consistent temporal relationship between fat with heparin injection and the fall in GH concentration, and the failure of either fat or heparin alone to consistently produce similar serum GH patterns suggest that the injection of fat with heparin with acute elevation of serum FFA concentrations to supraphysiologic levels caused virtual cessation of GH secretion. Thus, free fatty acids, in addition to glucose and amino acids, may be involved in the regulation of GH secretion.