Vasodilator therapy in pulmonary hypertension is limited by the lack of an agent selective for the pulmonary circulation. The effects of intravenous prostacyclin and two stable prostaglandin analogs, ZK 36-374 and CL 115,347, were assessed on the preconstricted pulmonary vasculature of the anesthetized dog. During hypoxic vasoconstriction ZK 36–374 (0.4 fig/kg per min) markedly reduced pulmonary artery pressure (26 ± 3 to 13 ± 1 mm Hg) (p < 0.05) and pulmonary vascular resistance (6.2 ± 1.1 to 2.8 ± 0.2 mm Hg/liter per min) (p < 0.01). There was no significant effect on cardiac output, aortic pressure or arterial blood gases. Pulmonary vasoconstriction induced by prostaglandin F2a was similarly affected by ZK 36–374, and in this instance the aortic pressure was also reduced (158 ± 11 to 129 ± 11 mm Hg) (p < 0.01). ZK 36–374 (0.2 jttg/kg per min) was more effective in lowering hypoxic pulmonary vascular resistance (from 6.5 ± 0.6 to 3.0 ± 0.3 mm Hg/liter per min) than was prostacyclin (0.75 /xg/kg per min) (from 6.3 ± 0.6 to 4.2 ± 0.4 mm Hg/liter per min) (p < 0.05) and resulted in a smaller fall in aortic pressure (p < 0.05). CL 115,347 (1.0 /xg/kg per min) had no effect on the pulmonary vasculature during normoxia or when preconstricted by prostaglandin F2a or hypoxia, but reduced aortic pressure and total systemic resistance (p < 0.05). It appears to be a selective systemic vasodilator with no pulmonary vascular activity. ZK 36-374 is an effective agent in reversing pulmonary vasoconstriction in this model and has potentially useful selectivity for the pulmonary circulation. Oral administration is possible.