Abstract When intact adipocytes of rat are briefly treated (5 min) with low concentrations (approx. K i of 5 · 10 −4 M) of sulfhydryl inhibitors, e.g., P-chlormercuribenzoate or HgCl 2, or reduced with dithiothreitol (> monothiols), the response of adenylate cyclase to isoproterenol or glucagon either in whole cells or in subsequently isolated membranes is severely decreased (approx. 60–80%) while the basal activity is relatively less affected, showing a decrease of −25 to −30%. This inhibition is reversible by appropriate treatment of the cells with low concentration 0.1–1 mM) of H 2O 2 which per se causes a partial increase (140–150%) in the basal adenylate cyclase activity and in the submaximal, but not maximal, hormone-response in isolated membrane when cells are treated with it for 3–5 min. The cellular content of cyclic AMP parallels, in general, with the adenylate cyclase response to hormones under the above treatments. The stimulation of adenylate cyclase by NaF added to cell-free membranes is not affected by pre-treatment of cells with either thiols, mercurials or H 2O 2. The evidence suggests that the hormonal stimulation of the adenylate cyclase-cyclic AMP system in these cells is impaired by either reduction or blockage of some labile sulfhydryl groups in a pre-catalytic site, while the available hormone-receptors or the catalytic site(s) remain unaltered. On the other hand, the hormonal elicitation of the adenylate cyclase-cyclic AMP system appears to involve an initial oxidation of these susceptible (external)-SH sites, which may be in the ‘coupling’ component. This type of regulation is distinct from the deactivation of the internal catalytic site by oxidants or electron acceptors, or its protection by thiols (Mukherjee, S.P. and Lynn, W.S. 1976 Fed. Proc. 35, 1702; Mukherjee, S.P. and Lynn, W.S. 1977 Arch. Biochem. Biophys, 184, 69; Mukherjee, S.P. and Lynn, W.S. 1979 Biochim. Biophys. Acta 568, 224).