Although the universal presence of thyroid hormones in vertebrates suggests a continuing role for them, at present no overall functional activity can be ascribed to these hormones. Thyroid hormone seems to play a role in metabolism regulation in different vertebrate groups. Some experimental evidence also is available suggesting a role of thyroid in the regulation of heart electrical activity. The action exerted by thyroid hormone-on heart rate in mammals has been shown for a long time. However, the mechanisms responsible for the mediation of such an action remain to be defined. Heart rate regulation by thyroid hormone seems to involve alterations in: i) sensitivity to autonomic mediators resulting from changes in density of cardiac receptors and/or in coupling of such receptors to the effector system; ii) electrophysiological properties of the myocardium resulting from changes in membrane currents. The above alterations, seen by experimentally modifying the thyroid state of animals, can also be shown in the case of physiological modifications of such a state associated to development, ageing or cold exposure. The changes in the cardiac receptor number and in membrane currents involving ionic pumping are mediated by mechanisms dependent on nuclear receptor binding and increases in protein synthesis. The changes involving specific ionic channels are likely mediated by both nuclear and extranuclear mechanisms. The possible role of the thyroid hormones in the regulation of fish cardiac electrical activity has not been investigated. In other vertebrate groups, information on the effects of altered thyroid function on cardiac activity, comparable to that on body and tissue metabolism, is lacking in the literature. Available data induce, however, to think that also in birds, amphibians and reptiles thyroid hormone might regulate electrical activity of heart through mechanisms similar to those operant in mammal species. However, it is appear that in poikilotherm species heart response to thyroid hormone is strongly affected by environmental temperature.