The present study describes the effects of halothane on morphine activity in the isolated left atria of the rat. Concentration-response curves were obtained for the negative inotropic effects of morphine on electrically stimulated left atria. Morphine significantly decreased the contractile force, with an inhibitory concentration 16 (IC16) of 3.130.698 +/- 22.5 X 10(-9) M. The opiate agonist was more potent in reserpinized rats, causing a consistent negative inotropic action over a wide range (10(-8)-10(-4) M) or morphine concentrations. The IC16 of morphine was significantly (P less than 0.001) decreased in the presence of 1.5% v/v halothane. The administration of L-naloxone (3 X 10(-7)-10(-6) M) but not D-naloxone (10(-6) M) antagonized the inhibitory effects of morphine in the presence of halothane. These results demonstrate that halothane increases the potency of morphine on the isolated left atria and suggest that this effect is mediated by opioid receptors.