Endothelium-derived nitric oxide (NO) inhibits in vitro platelet aggregation via a cGMP-dependent mechanism. The effect of inhaled NO an platelet-mediated pulmonary thrombosis following intravenous thrombotic challenge with collagen was examined in rats and compared with the effect of G4120, a cyclic Arg-Gly-Asp-containing synthetic pentapeptide that binds to the platelet glycoprotein IIb/IIIa receptor. Intraplatelet cGMP dose-dependently increased from 39+/-6 fmol/10(8) platelets in control to 46+/-6, 68+/-13, and 81+/-13 fmol/10(8) platelets after inhalation with 20, 40, and 80 ppm NO, respectively (P<.05 for 40 and 80 ppm). Ex vivo platelet aggregation of platelet-rich plasma induced by 1 mu g/mL collagen was reduced from 75+/-4% in control rats to 22+/-10% and 20+/-7% in rats ventilated with 40 and 80 ppm NO, respectively, and to 30+/-9% in G4120-treated rats (each P<.05 versus control). Circulating platelet counts 3 minutes after collagen injection were significantly higher in the inhaled NO and G4120 groups compared with control rats (250 000+/-18 000 and 223 000+/-10 000/mu L versus 160 000+/-18 000/mu L, each P<.05). The rise in pulmonary arterial pressure after collagen injection was significantly reduced in NO- and G4120-treated rats (26+/-1 and 27+/-1 versus 32+/-1 mm Hg in control rats, each P<.05). The number of pulmonary resistance vessels containing platelet thrombi was significantly smaller after inhaled NO and G4120 treatment compared with control (56+/-3% and 50+/-3% versus 68+/-3%, respectively; P<.05. Thus, NO inhalation reduces in vivo activation of circulating platelets and platelet-rich thrombosis in thromboembolic pulmonary hypertension. Inhalation of NO may be useful in cardiovascular diseases associated with platelet activation.