Résumé : The use of high concentrations of inhaled oxygen has been associated with adverse effects but recent data suggest a potential therapeutic role of normobaric hyperoxia (NH) in sepsis and cerebral ischemia. Hyperoxia may induce vasoconstriction and alter endothelial function, so we evaluated its effects on the microcirculation in 40 healthy adult volunteers using side-stream dark field (SDF) video-microscopy on the sublingual area and near-infrared spectroscopy (NIRS) on the thenar eminence. In a first group of volunteers (n=18), measurements were taken every 30min: at baseline in air, during NH (close to 100% oxygen via a non-rebreathing mask) and during recovery in air. In a second group (n=22), NIRS measurements were taken in NH or ambient air on two separate days to prevent any potential influence of repeated NIRS measurements. NH significantly decreased the proportion of perfused vessels (PPV) from 92% to 66%, perfused vessel density (PVD) from 11.0 to 7.3vessels/mm, perfused small vessel density (PSVD) from 9.0 to 5.8vessels/mm and microvascular flow index (MFI) from 2.8 to 2.0, and increased PPV heterogeneity from 7.5% to 30.4%. Thirty minutes after return to air, PPV, PVD, PSVD and MFI remained partially altered. During NH, NIRS descending slope and NIRS muscle oxygen consumption (VO2) decreased from 8.5 to 7.9%/s and 127 to 103 units, respectively, in the first group and from 10.7 to 9.4%/s and 150 to 115 units in the second group. NH, therefore, alters the microcirculation in healthy subjects, decreasing capillary perfusion and VO2 and increasing the heterogeneity of the perfusion.