Intracellular microelectrodes were used to study the electrophysiological effects of morphine on guinea-pig papillary muscle. Morphine (5 x 10(-4) M) caused a significant decrease in the maximum rate of depolarization. At high concentrations (5 x 10(-3) M) morphine induced a decrease in the action potential amplitude and a prolongation of the action potential duration. The administration of naloxone (10(-7) M) partially antagonized the cardiac electrophysiological effects of morphine. These results suggest that the electrophysiological effects of morphine may be due to an interaction with opioid receptors.