In previous work, morphine sulfate was shown to decrease heart rate (HR) and cardiac output (CO) in a dose-related fashion. It was hypothesized that this effect was mediated by opiate receptors located in the myocardium. The present study evaluated the effect of opiate receptor antagonism with naloxone using a modified Langendorff rat heart perfusion apparatus. Sixty-five rat hearts were excised and perfused with Krebs-Henseleit buffer (KHB) solution, to which morphine sulfate and naloxone (NAL) were added in different concentrations. In the initial studies, NAL (10(-5) M) was added to the perfusate prior to the incremental additions of morphine. This resulted in no antagonism of the previously described opiate agonist effects. Norepinephrine (NE; 10(-9) M) was then added to the perfusate prior to the NAL or morphine. The NE did not affect the dose-related decrease in HR and CO when morphine was added but did permit the attenuation of the morphine effect by the addition of increasing concentrations of NAL up to 10(-5) M. These results suggest that the agonist effect can be attenuated by opiate receptor antagonism with NAL; the data also suggest a possible interrelationship between opiate and catecholamine receptor activity in the myocardium.