Cholera toxin produces its pathological effects via stimulation of the intestinal adenylate cyclase (ATP pyrophosphate-lipase (cyclizing) EC 4.6.1.1), thus causing an increase in the level of 3',5'-AMP in intestinal epithelial cell. Activation of cyclase in cell-free systems requires the A promoter of the toxin but not the B or binding subunit and is dependent on NAD, ATP and a cytosol factor(s). It has been demonstrated that hormone stimulation of adenylate cyclase of avian erythrocytes is dependent on GTP binding at a regulatory site. Substitution for GTP of a hydrolysis-resistant analog, guanosine 5' (beta-gamma-imino) triphosphate, results in maximal and persistent activation of adenylate cyclase. It has been reported that turkey erythrocytes contain a guanosine triphosphatase (GTPase) with a very low Km for GTP that can be activated by catecholamines. This activated GTPase is inhibited by cholera toxin. To evaluate the hypothesis of Cassel and Selinger that this GTPase is a regulatory moiety of the cyclase complex, we are examining in parallel the effects of catecholamines, toxin and other variables on GTPase and adenylate cyclase activities in rat brain particulate and turkey erythrocyte membranes.