Halothane stimulated basal adenylate cyclase activity in rat cardiac membranes. Maximal stimulation (54%) was obtained after equilibrating the membranes with 2% halothane. Halothane did not affect the fractional stimulation of adenylate cyclase activity produced by either forskolin or isoproterenol. However, halothane decreased carbamylcholine inhibition of adenylate cyclase activity stimulated by both forskolin and isoproterenol. Maximal depression of carbamylcholine inhibition of stimulated cyclase activity was obtained after equilibration with 1% halothane. These results are consistent with evidence from ligand binding studies and indicate that halothane disrupts muscarinic receptor-G-protein interactions.