1. Isolated rat hearts accumulated 102 pmol/g wet wt/min of isoprenaline when perfused for 5 min with 0-6 muM (+/-)-3H-isoprenaline. 2. The 3-methoxy derivative of isoprenaline ('methoxy isoprenaline') (10 muM) significantly inhibited this uptake by 57%, metanephrine (10 muM) by 29% and normetanephrine (10 muM) by 21%. 3. (+/-)-Isoprenaline (0-6 muM) infused into isolated perfused rat hearts for 5 min activated glycogen phosphorylase 2-4-fold. Normetanephrine (10 muM) or metanephrine (10 muM included in the perfusate significantly potentiated this activation, but 3-0-methyl isoprenaline (10 muM significantly reduced it. However, 3-0-methyl isoprenaline potentiated the ability of 4-8 muM isoprenaline to stimulate phosphorylase. 4. Neither metanephrine (10 muM) nor normetanephrine (10 muM) altered peak inotropic responses to injections of (+/-)-isoprenaline into the solution perfusing isolated rat hearts. 3-0-methyl isoprenaline (10 muM) shifted the isoprenaline dose-response curve to the right, but did not affect the inotropic responses to CaCl2, confirming that 3-0-methyl isoprenaline possess beta-adrenoceptor antagonist activity. 5. Inotropic responses to isoprenaline were significantly prolonged by both 3-0-methyl isoprenaline and normetanephrine (10 muM). 6. These results indicate that blockade of extraneuronal accumulation of catecholamines causes potentiation of both metabolic and mechanical beta-adrenoceptor-mediated responses of the heart to isoprenaline. It is suggested that Uptake2 and the cardiac beta-adrenoceptor are separate entities, and that the beta-adrenoceptor is localized in the sarcolemma. The physiological function of Uptake2 may be to help clear the sympathetic synaptic gap of liberated neurotransmitter.