The effects of two isoforms of human endothelin (ET) on the pulmonary and systemic vascular beds were compared in the anesthetized intact-chest rabbit under conditions of constant pulmonary blood flow and left atrial pressure. Intralobar bolus injections of ET-1 (0.1-1 micrograms) and ET-3 (1-3 micrograms) produced modest vasoconstriction in the pulmonary vascular bed, whereas both peptides decreased systemic arterial pressure. The pulmonary vasoconstrictor response to ET-1 and ET-3 was inhibited by intralobar infusion of nitrendipine but was not altered by indomethacin. In contrast to the small effects of ET-1 and ET-3 on intact pulmonary resistance vessels, both peptides markedly contracted isolated pulmonary conductance vessels, with greater activity on venous than on arterial segments. Intravenous bolus injection of ET-1 (0.1-0.3 micrograms) or ET-3 (0.3-1 microgram) decreased systemic arterial pressure, increased cardiac output, and markedly decreased systemic vascular resistance. Higher doses of ET-1 produce a biphasic systemic vascular response with a prominent secondary pressor component. The present data suggest that the pulmonary vasoconstrictor activity of ET-1 is greater than that of ET-3 and their pressor activity depends on an extracellular source of calcium. The pulmonary and systemic hemodynamic effects of ET-1 and ET-3 in the rabbit do not depend on cyclooxygenase products. The systemic vasodilator response to ET-1 is not altered by first-pass lung transit. Furthermore the systemic vasodilator response to both peptides occurs independent of activation of muscarinic, beta 2-adrenergic, and platelet-activating factor receptors. Although ET-1 and ET-3 were initially reported as vasoconstrictor peptides, the present data suggest that, by having unique and potent systemic vasodilator activity, ET-1 and ET-3 act differently in the systemic and pulmonary vascular beds under resting conditions in the rabbit.