The ability of the Beta1/Beta2 partial agonist carteolol to act as an agonist on Beta3-adrenergic receptors was investigated by studying its ability to stimulatethermogenesis (oxygen consumption) in brown fat cells isolated from hamsters. Carteolol wasable to induce thermogenesis, with an EC50 of 5 µM, but it was only a partial(40%) agonist. D,L-Propranolol had a pKB of 5.1 as anantagonist against carteolol in this system, indicating that the carteolol effect was probablymediated via Beta3-receptors. Also in mouse and rat cells, carteolol was a partial agonist withEC50 values around 1 µM. Being a partial agonist, carteolol acted as an antagonistagainst norepinephrine-, BRL-37344-, or CGP-12177-stimulated thermogenesis with apKB of approximately5. The partial agonist effects of carteolol are discussed in relation to the absence of agonisteffect of this compound on guinea-pig taenia cecum Beta3-receptors and in relation to the possible plurality of Beta3-receptors.Key words: carteolol, Beta3-adrenergic receptor, brown fat cells, nonshivering thermogenesis.