The effects have been studied of human growth hormone (HGH) and of prolonged fasting on the fatty acid composition in serum of free fatty acids (FFA), triglycerides, cholesteryl esters and phospholipids. The serum fatty acid pattern was also studied in 3 acromegalic women.Following one injection of HGH, serum FFA concentrations rose and were highest at 8 h. This was mainly due to increased free oleic acid. Serum phospholipid concentrations fell and were lowest at 4 h.Prolongation of an overnight fast by 8 h also induced significant increases in serum FFA concentrations with an increase in free oleic acid, but of a smaller magnitude than after HGH injection; decreases occurred in free stearic and linoleic acids. The percentage level of phospholipid palmitate increased and that of linoleate decreased. In addition, the percentage level of triglyceride stearate decreased.When HGH was administered over 3 days, fasting serum FFA increased only on the 2nd day of treatment. The concentrations of serum phospholipid decreased steadily. An increase occurred in the percentage levels of triglyceride palmitate, cholesteryl oleate and phospholipid linoleate and eicosatrienoate. A decrease occurred in the percentage level of phospholipid oleate.Acromegalic patients had higher percentage levels of saturated acids in free and triglyceride fatty acids compared with normal controls. The high percentage of triglyceride saturated acids was the only characteristic that increased endogenous secretion and exogenously administered HGH had in common.