The calmodulin binding domain of type I adenylyl cyclase, which contains cysteine residues, was found to be affected by sulfhydryl reagents, with nitric oxide and o-iodosobenzoate inhibiting calmodulin stimulation of the enzyme. The loss of calmodulin sensitivity due to these reagents was reversible by reducing agents, suggesting that oxidation of critical cysteine residues in the calmodulin-binding region of the type I adenylyl cyclase impairs enzyme function.