Serum and plasma obtained from fasted rats were examined for their capacity to promote glycerol release from isolated fat cells of the rat,in vitro. Dose-response curves indicated that the lipolytic potencies of serum and plasma were very similar. Plasma comparison experiments suggested that lipolysis promoted by this circulating agonist had a requirement for calcium ions. Since adrenaline can enhance lipolysis in a calcium-free medium, the lipolytic capacity of serum and plasma could not be simply explained in terms of circulating catecholamines. The concentrations of glycerol or free fatty acids did not increase when serum or plasma were incubated in the absence of fat cells.