Lipolysis in rat adipocytes is controlled by the hormonally mediated stimulation and inhibition of adenylate cyclase. This dual regulation involves stimulatory (Gs) and inhibitory (Gi) GTP-binding proteins which control cAMP production in a GTP dependent manner. Adenosine, acting via the A1 receptor-Gi complex provides tonic regulation of adenylate cyclase and lipolysis in rat adipocytes. Adipocytes prepared from young obese Zucker (fa/fa) rats exhibit less stimulation (or greater inhibition) in response to adenosine deaminase, alone or in combination with lipolytic hormones, as compared with their lean littermates. Adenylate cyclase, measured in membranes prepared from obese adipocytes, showed decreased sensitivity to activation by low concentrations of GTP and was not inhibited by higher concentrations of the guanine nucleotide which, in lean control rats results in a biphasic activity curve. Adenosine A1 receptor binding, measured in these same membranes, demonstrated an increased sensitivity to activation by the GTP analogue, guanylyl imidodiphosphate. The presence of the analogue results in the dissociation of the receptor-Gi complex and conversion to the low affinity form in a greater proportion of receptors in the obese membranes. These results are consistent with an increased sensitivity to adenosine mediated inhibition of adenylate cyclase and lipolysis in the fat cells of the young obese (fa/fa) Zucker rat.