Glucagon, vasoactive intestinal polypeptide and secretin are strong stimulators of lipolysis in adipose tissue from laboratory animals. Yet, in human adipose tissue these data could not be confirmed under comparable experimental conditions. Using pH stat titration, an advanced in vitro test system for evaluating lipolysis, it was possible to demonstrate lipolytic activity for glucagon down to a concentration of 10(-8) mol/l. This is comparable to the minimal effective doses in rat adipose tissue and corresponds to the effect of equimolar concentrations of noradrenaline in man. Secretin with an amino acid sequence very similar to glucagon was not lipolytically active, while VIP stimulated free fatty acid release in a concentration of 10(-6) mol/l. Since the minimal effective dose of glucagon is only 30 times greater than the plasma levels a physiological significance of these finding may be suggested. The lipolytic activity of VIP seems to be only of pharmacological interest.