The influence of several agents known to affect the rate of lipolysis or the phosphodiesterase (PDE) activity in rat adipose tissue was studied in human adipose tissue in vitro. Only insulin, catecholamines, prostaglandin E1, and cAMP increased PDE activity in man (by 20-40% over the control values). In dose-response studies, it was found that the ED50 values for insulin and isopropyl noradrenaline were of the same magnitude as the previously observed ED50 values for lipolysis in human adipose tissue. The time course for insulin and isopropyl noradrenaline showed maximum stimulation after 10 min, followed by a decline, with the curve approaching zero at 60 min. In subcellular fractions of adipose tissue, the highest specific PDE activity was found in the particulate fraction, in which the effects of both insulin and isopropyl noradrenaline were most marked. It is concluded that the stimulatory effect of catecholamines on PDE may be one reason for the failure of these agents to produce a sustained increase in the cAMP level and that the effect of insulin on PDE may be one mechanism by which insulin reduces the rate of lipolysis.