text: Ginkgo biloba, an extract of the ginkgo tree, may prevent or lessen symptoms of acute mountain sickness in humans. The mechanism of this effect is poorly understood. One hypothesis is that ginkgo alters nitric oxide (NO) metabolism, possibly by scavenging NO or altering nitric oxide synthase expression and thereby lessening the vasodilatory effects of NO. To date, an effect of Ginkgo biloba on NO metabolism has not been demonstrated in humans. We measured exhaled nasal NO output in humans (n = 9) during normoxia and then during acute normobaric hypoxia (goal oxyhemoglobin saturation 75% to 85%) before and after administration of a standardized extract of Ginkgo biloba (120 mg twice daily for 5 days). Oxygen saturation, heart rate, and minute ventilation were similar before and after Ginkgo biloba administration. Exhaled nasal NO output was increased during normoxia following ginkgo (p < 0.02) and reduced during normobaric hypoxia both before (p < 0.02) and following (p < 0.003) ginkgo. Exhaled nasal NO output during normobaric hypoxia was lowest following ginkgo (p < 0.003). We conclude that Ginkgo biloba increases exhaled nasal NO output during normoxia and enhances reduced exhaled nasal NO output during normobaric hypoxia. Our results suggest that Ginkgo biloba may act to reduce AMS through an effect on NO metabolism.