In the experiments, using isolated, perfused according to Langendorff heart of guinea pig, was investigated the effect of flokalin, a new fluor-containing opener of ATP sensitive channels (K(ATP)), upon the functioning of the heart. It was shown, that flokalin in a dose dependent manner dilates coronary vessels, decreases final diastolic pressure in the left ventricle and the rate of extrasystoles. Flokalin does not significantly affect the developing pressure in the left ventricle, max and min dP/dt, the frequency of cardiac contractions. 5-hydroxydecanoic acid (5-HD), an inhibitor of KATP in the mitochondrial membranes, increases the developing pressure in the left ventricle, max and min dP/dt, perfusion pressure in the coronary vessels. Simultaneously the final diastolic pressure in the left ventricle is decreased. Inhibition of K(ATP) channels in the mitochondrial membrane does not affect the vasodilatory effects of flokalin in the coronary vessels. This data, under the condition of specific action of 5-HD, permits us to suppose the participation of sarcolemmal KATP channels in the mentioned processes. It was shown that inhibition of mitochondrial K(ATP) channels results in decrease of the final diastolic pressure in the left ventricle and can enhance the effect of flokalin upon the final diastolic pressure, thus urging the idea of the leading role of sarcolemmal K(ATP) channels in the mentioned above processes.