T2*-weighted gradient echo magnetic resonance images of rat brain were obtained dynamically during acute hypoxemic hypoxia to investigate the relations between changes in cerebral blood oxygen saturation (deltaYb), blood hematocrit (Hct), and R2* (deltaR2*). Images from hypoxemic rats with normal Hct (42.8%+/-2.33%; n=12) were compared with those from hypoxemic rats with mild (33.4%+/-1.88%; n=8) or moderate (27.14%+/-2.7%; n=10) reduction of Hct. A linear relation between deltaYb and deltaR2* was obtained for all three groups. However, the slopes of the linear regressions were statistically different from one another (P < 0.001), with the slopes of the regression lines increasing inversely with Hct; that is, the slope for normal Hct is less than the slope for mildly reduced Hct, which is less than the slope for moderately reduced Hct. These data suggest that for any given reduction in the oxygen saturation of cerebral blood, the deltaR2* will be of a lesser magnitude when the hemoglobin concentration is reduced; the data are consistent with existing theoretical models of deoxyhemoglobin content-dependent effects in T2*-weighted magnetic resonance imaging.