OBJECTIVE: To study the effect of nitric oxide (NO) inhalation on platelet aggregation in neonates with persistent pulmonary hypertension. METHODS: The platelet aggregartion rate with the induction of epinephrine, collagen and adenosine diphosphate (ADP) was assayed by turbidimetry 6 hours after NO inhalation in 11 neonates with persistent pulmonary hypertension (NO inhalation group). Twelve neonates with this disorder receiving oxgen inhalation served as the controls. RESULTS: The platelet aggregation rate with the induction of epinephrine, collagen and ADP was significantly inhibited in the NO inhalation group compared with the controls: (0.39 ±0.11)% vs (0.48±0.10)%, (0.41±0.19)% vs (0.69 ± 0.22)%, (0.32±0.12)% vs (0.50 ± 0.16)%, respectively ( P <0.05 or 0.01). CONCLUSIONS: NO inhalation may restrain platelet aggregation and increase hemorrhage risk.