The properties of the β₁-/β₂-adrenoceptor partial agonist carteolol were investigated in atypical β-adrenoceptors on the guinea pig gastric fundus. Carteolol induced concentration-dependent relaxation in this tissue (pD₂=5.55, intrinsic activity=0.94). However, a combination of the selective β₁-adrenoceptor antagonist atenolol (100 μM) and the selective β₂-adrenoceptor antagonist butoxamine (100 μM) produced only small rightward shifts in the concentration-response curves of carteolol in the gastric fundus (pD₂=4.91, intrinsic activity=0.94). In the presence of both atenolol (100 μM) and butoxamine (100 μM), the non-selective β₁-, β₂- and β₃-adrenoceptor antagonist (±)-bupranolol (10-100 μM) caused a concentration-dependent right-ward shift of the concentration-response curves for carteolol in the guinea pig gastric fundus. Schild plot analyses of the effects of (±)-bupranolol against carteolol gave the pA₂ value of 5.29 and the Schild slope was not significantly different from unity. Furthermore, carteolol (10 μM) weakly but significantly antagonized the relaxant responses to catecholamines ((−)-isoprenaline, (−)-noradrenaline and (−)-adrenaline), a selective β₃-adrenoceptor agonist BRL37344 ((R^*, R^*)-(±)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid sodium salt) and a non-conventional partial β₃-adrenoceptor agonist (±)-CGP12177A ([4-[3-[(1, 1-dimethylethyl)amino]-2-hydroxypropoxy]-1, 3-dihydro-2H-benzimidazol-2-one] hydrochloride) in the guinea pig gastric fundus. These results suggest that the partial agonistic effects of carteolol are mediated by atypical β-adrenoceptors in the guinea pig gastric fundus.