1. Forskolin, an activator of adenylate cyclase, potentiated the relaxing response to isoproterenol in rabbit aortic rings precontracted by phenylephrine (PE).2. The potentiating effect of forskolin was inhibited by propranolol, a β-adrenoceptor inhibitor, but not by methylene blue, a guanylate cyclase inhibitor.3. The relaxing response to terbutaline, a β 2 -adrenoceptor agonist, but not lower concentrations of dobutamine, a β 1 -adrenoceptor agonist, also was potentiated by forskolin. Forskolin, however, potentiated the relaxing response to high concentrations of dobutamine, which activates both β 1 - and β 2 -adrenoceptors.4. Yohimbine, an α 2 -adrenoceptor inhibitor, glyburide, an ATP-sensitive K + channel inhibitor, iberiotoxin, a Ca 2 + -activated K + channel inhibitor, or endothelium-removal failed to affect the potentiating effect of forskolin.5. Dibutyryl cyclic AMP (cAmp) also potentiated the relaxing response to terbutaline.6. These results suggest that in rabbit aortic rings forskolin causes the apparent potentiation of isoproterenol-induced relaxation by mainly affecting the relaxing response due to the activation of β 2 -adrenoceptors by the forskolin-induced increase in the level of cAMP.