The CO2 fraction of an anesthetized rat's breathing mixture was changed (from 0 to 10%) to attempt to change the brain microcirculation and observe these changes in diffusion measurements of the neural tissue. Brain apparent diffusion coefficients were measured to be (0.71 +/- 0.01) X 10(-3) mm2/s before sacrifice and (0.39 +/- 0.01) X 10(-3) mm2/s after sacrifice. Multiple diffusion components were observed, consistent with flowing material, but the extra components did not increase with increased CO2. It is proposed that the additional components may be due to extracellular, extravascular water such as CSF.