Metabolism of catechol amines was studied by fluorometric determinations of adrenaline, noradrenaline, dopamine, and dopa in the dog, rat and human hearts. The distribution in cardiac chambers, and biosynthesis and uptake of these amines suggested rather specific metabolism in this organ. Some aspects on uptake-binding-release mechanism of catechol amines was obtained under the pathological conditions, and also by the administration of catechol anines, MAO-inhibitors and reserpine.