Pulmonary vascular responses to leukotriene D4 were investigated in the intact-chest animal under conditions of controlled pulmonary blood flow. Intralobar injections of leukotriene D4 in the sheep caused dose-dependent increases in lobar arterial and small vein pressures without influencing left atrial or systemic arterial pressure. Leukotriene D4 was very potent in increasing pulmonary vascular resistance in the sheep, with activity similar to that of U-46619, a thromboxane A2 mimic. Pulmonary vascular responses to leukotriene D4 in the sheep were similar when the lung was ventilated and when lobar ventilation was arrested. Responses to leukotriene D4 were similar when the lung was perfused with blood or with dextran. Pulmonary vascular responses to leukotriene D4 but not U-46619 in the sheep were reduced by inhibitors of cyclooxygenase and thromboxane synthesis. In contrast, leukotriene D4 had modest pressor activity in the pulmonary vascular bed of the cat whereas U-46619 had marked activity in this species. Responses to leukotriene D4 in the cat were not altered by cyclooxygenase inhibitors. It is concluded that leukotriene D4 has marked pulmonary vasoconstrictor activity in the sheep, increasing pulmonary vascular resistance by constricting intrapulmonary veins and upstream segments. In this species, responses to leukotriene D4 were independent of changes in ventilation or interaction with formed elements but were dependent on the formation of cyclooxygenase products including thromboxane A2. However, in the cat, leukotriene D4 had very modest pressor activity, and this activity was not dependent on the integrity of the cyclooxygenase pathway. These data suggest considerable species difference in responses to leukotriene D4, a major component of the slow-reacting substance of anaphylaxis, in the pulmonary vascular bed.