To determine the site of inhaled nitric oxide (NO)-induced pulmonary vasodilation, a double vascular occlusion technique was used with rabbit lungs ventilated and perfused at 20 ml/min with Krebs solution containing 3% dextran and 30 microM indomethacin. Inhaled NO (120 ppm for 3 min) reduced pulmonary vasoconstriction produced by U-46619 infusion (0.5-1.2 nmol/min), significantly decreasing total resistance (RT) [1,080 +/- 51 (SE) vs. 1,545 +/- 109 mmHg.l-1.min; P < 0.01]. Acetylcholine infusion (ACh; 2-5 nmol/min) and nitroglycerin (NTG; 0.35 mumol) likewise decreased RT. Arterial resistance (Ra) was also significantly less with inhaled NO, ACh, and NTG compared with U-46619 alone. Venous resistance (Rv), however, was unchanged. When the direction of perfusion was reversed in the lung, inhaled NO, ACh, and NTG significantly decreased RT compared with U-46619 alone, and Rv was also reduced by all three agents. After electrolysis-induced acute lung injury, inhaled NO significantly reduced both RT and Ra compared with U-46619 alone, whereas Rv was unaffected. Our results demonstrate that inhaled NO gas affects primarily the arterial (precapillary) component of the pulmonary circulation but, under conditions of extreme venous constriction, may dilate the postcapillary component as well.