Near-infrared spectroscopy was used to measure global cerebral blood flow and volume in 10 healthy adult volunteers. High- and low-cerebral blood flow compartments were detected with mean flows for all 10 subjects of 59 +/- 21 (SD) and 11 +/- 4 ml.100 g-1.min-1, respectively. The mean cerebral blood volume of the group was 2.85 +/- 0.97 ml/100 g. Analysis of spontaneous changes in the cerebral concentrations of oxyhemoglobin and deoxyhemoglobin demonstrated strong correlations between respiratory rate and the oscillation frequency of cerebral oxyhemoglobin concentration (r = 0.99) and arterial oxygen saturation (SaO2) (r = 0.99). An estimate of the mean cerebral oxygen saturation for all subjects averaged 59.4 +/- 12.4% when their mean SaO2 was 91.8 +/- 2.4% (equivalent to 67.6 +/- 13.8% at a normoxic SaO2 of 98%). These results demonstrate that near-infrared spectroscopy can be used as a noninvasive bedside technique for both qualitative and quantitative evaluation of cerebral hemodynamics and oxygenation in adults.