The similarity in the chemical structures and molecular sizes of the polypeptides glucagon, secretin and a recently isolated vasoactive intestinal polypeptide, VIP, prompted an investigation of the comparative effects of these hormones on lipolysis in rat and chicken fat cells. VIP and secretin had no stimulatory effect upon lipolysis in isolated chicken fat cells, whereas glucagon was active in concentrations below 1 ng/ml. VIP and secretin had no inhibitory effect on the glucagon-stimulated lipolysis in chicken fat cells. All three hormones stimulated lipolysis in the rat system. VIP activated adenyl cyclase in fat cells isolated from rat epididymal fat pad, but had no stimulatory effects upon the enzyme from chicken gizzard fat. VIP showed no interference with the stimulatory effect of glucagon on the adenyl cyclase in any of the fat cell systems.