Near infrared optical spectroscopy (NIR) is a recognized technique for the study of the cellular oxygen metabolism, and oxygen delivery to the tissue. During the last few years, this technique has become useful for the study of the regional cerebral oxygen saturation (rSO2); however, up to date no report exists showing the values of the normal variability of the rSO2 in humans. In the present study, rSO2 was measured via in vivo optical spectroscopy (INVOS 2910, Somanetics, Troy, MI) in 100 randomly selected adults (aged 16 to 78), who displayed no symptoms nor clinical signs of cerebral disorder. The sample population consisted of 51 males and 49 females (16 to 78 years old); 27 had dark skin and 73 had light skin. The mean rSO2 found was 68.6 5.6. The relationship between mean rSO2 and age was studied using the regression analysis. No association between age and rSO2 was found (p = 0.132). The correlation of mean rSO2 with sex and with color were evaluated by a two sample t-test for each. There was no significant difference between the mean rSO2 for females and the mean rSO2 for males (p =0.132). However, the mean rSO2 for light skin individuals was significantly higher than the mean rSO2 for dark skin individuals (rSO2 =69.50 forlight skin, rSO2 =66.15 for dark skin; p =0.0078). The advantages of infrared spectroscopy are evident; this is a noninvasive, simple, relatively inexpensive technology, easily interpreted and suitable for continuous monitoring of rSO2 at bedside or during brain surgery. We consider NW infrared spectroscopy the best actual technique for early detection of acute cerebral hypoxia, and a method for anticipation of undesirable outcome in these patients.