We have studied the effect of Р-endorphin (an endogenous opioid peptide) on the functional characteristics of isolated rat heart. In physiological concentrations (6.3 X 10-10 M) Р-endorphin induces a negative inotropic effect, accompanied by a slight decrease in the heart rate, and has no effect on the perfusion pressure of the coronary vessels. It is shown that the effect of Р-endorphin is mediated by 5-opioid receptors. ATP-dependent K+-channels contribute the greatest contribution to the implementation of the effect of Р-endorphin on the isolated heart of a rat. The role of voltage-dependent K+-channels and NO-mediated mechanisms in the implementation of the effects of Р-endorphin is minor.