The level of plasma free fatty acids (FFA) in both laying and immature non-laying hens was not markedly decreased by intravenously injected amounts of glucose sufficient to double the level of plasma glucose for 15–30 min. This was true for fasting and for fed birds. Adrenocorticotrophic hormone (ACTH) given i.v. to laying birds produced a definite increase in the levels of plasma FFA and glucose in doses of 60 i.u./kg. Daily doses of long-acting ACTH given i.m. for 3 days to laying birds produced a marked increase in plasma glucose accompanied by a pronounced fall in plasma FFA. Insulin markedly lowered the plasma glucose level in both mature and immature birds and caused a large and immediate increase in plasma FFA. Glucagon also induced a marked increase in plasma FFA accompanied by a rise in plasma glucose. The effects of insulin and glucagon were not abolished by pretreating the birds with reserpine or hexamethonium bromide. It is concluded that the effect of glucagon on plasma FFA is probably a result of a direct action on avian adipose tissue and that insulin may promote a similar response by an increased release of glucagon from the pancreas.