Oxygen free radicals are produced during stress, are unstable, and potentially interact with other cellular components or molecules. This reactivity can influence cellular function, including a prolongation in tissue recovery following exercise. We tested the effect of ribose (d-ribose), a pentose carbohydrate, in a double-blinded, crossover study on markers of free radical production during hypoxic exercise. Seven healthy volunteers cycled at their lactate threshold for 25 minutes while inhaling 16% O(2) with a subsequent 60-minute resting period at room air. Subjects ingested either placebo or 7 g of ribose in 250 mL of water before and after the exercise session. Urinary malondialdehyde (MDA) and plasma reduced glutathione levels increased significantly during placebo ingestion (0.2 +/- 0.03 nM/mg and 0.26 +/- 0.29 microM, respectively) but were lower with ribose supplementation (0.04 +/- 0.03 nM/mg and 0.38 +/- 0.29 microM, respectively; P < .05). Uric acid levels were similar between groups (ribose vs. placebo, 4.55 +/- 0.06 mg/dL vs. 4.67 +/- 0.06 mg/dL). Ribose demonstrated a beneficial trend in lower MDA and reduced glutathione levels during hypoxic stress.