Insulin is known to act in adipocytes as a lipogenic and antilipolytic hormone. The biochemical mechanisms underlining the action of insulin, however, remain relatively obscure. Recently, it has been shown that insulin causes the release of a low molecular weight compound(s) insulin mediator) through its activation of a protease. This compound(s) has been shown to activate glycogen synthase phosphatase and pyruvate dehydrogenase phosphatase, and inhibit cAMP-dependent protein kinase. It appears to serve as a mediator of insulin action. Different actions of insulin may be mediated by different mediators. We have developed a bioassay for the apparent mediator(s) in which insulin can be used to construct a calibration curve for quantitation of the mediator. Our experimental results strongly support the idea that antilipolytic action of insulin is mediated through its cAMP-lowering ability in adipocytes. Through the use of the Sutherland criteria, to define hormone messengers, we shall, first, elucidate the chemical identitiy of the cAMP-lowering mediator, thereby establishing an unbiased measurement of the activity. Secondly, the mechanism of action of the mediator of several cAMP related enzymes (low Km cAMP phosphodiesterase, adenylate cyclose, CAMP-dependent protein knase and phosphorprotein phosphatase) will be studied. Thirdly, the metabolic pathway of this mediator(s), the enzymes, the substrates, and the intermediates in relation to the formation and degradation of the mediator(s), will be thoroughly researched. At the present time we have demonstrated that the first and fourth criteria of Sutherland for a hormone messenger apply in the case of the mediator. We are very interested in this cAMP-lowering mediator(s) of insulin for the opportunities that it may provide in the better understanding of insulin action and hope that this investigation will open new avenues of research.