Carazolol is a beta1/beta2 adrenoceptor antagonist of high potency used in the treatment of hypertension. Its affinity for the beta 3-adrenoceptor was determined in Chinese hamster ovary cells transfected with the gene of the human or the murine beta 3-adrenoceptor. Carazolol is recognized with a nanomolar affinity, which positions it among the best ligands for beta 3-adrenoceptors. The adenylyl cyclase stimulation was measured in transfected cells where carazolol acted as a full agonist on both murine and human receptor subtypes. Furthermore, in murine adipocyte-like 3T3-F442A cells, which express beta 3-adrenoceptor naturally, carazolol induced lipolysis. This compound also appeared to be a useful tool for molecular characterization of the beta 3-adrenoceptor, unlike the classical beta 3-adrenoceptor agonists, carazolol conferred an appreciable protection of receptor binding sites against inactivation by the reducing agent dithiothreitol. The major iodinated analog of carazolol retained its binding characteristics for the beta 3-adrenoceptor and remained an efficient adenylyl cyclase stimulator in cells expressing human beta 3-adrenoceptor.