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A novel noninvasive magnetic resonance imaging (MRI) method was developed to determine in vivo blood oxygen saturation and its changes during motor cortex activation in small cerebral veins. Specifically, based on susceptibility measurements in the resting states, pial veins were found to have a mean oxygen saturation of Y_rest = 0.544 ± 0.029 averaged over 14 vessels in 5 volunteers. During activation, susceptibility measurements revealed an oxygen saturation change of ΔY_susc = 0.14 ± 0.02. Independent evaluation from blood flow velocity measurements yielded a value of ΔY_flow = 0.14 ± 0.04 for this change. These results validate the blood oxygenation level‐dependent (BOLD) model in functional MRI (fMRI). Hum. Brain Mapping 5:341–346, 1997. © 1997 Wiley‐Liss, Inc.