Opioid dermorphin induced a negative inotropic effect on the isolated perfused by Straube frog's heart, this effect was blocked by naloxone. On the background of dermorphin negative inotropic effect acetylcholine inhibited the ventricular contractile activity to the same degree as in the control experiments before dermorphin injection. But after the combined infusion of naloxone and dermorphin removed the opioid inotropic effect, the negative inotropic effect of acetylcholine became significantly weaker than in the control. It has been concluded that there are opiate receptors in the frog's ventricular myocardium, their activation leads to the negative inotropic effect. Dermorphin may act like atropine on the inotropic effects of acetylcholine, this action doesn't depend on the opiate receptors activation.