The relation between cerebral blood flow (CBF) and cerebral oxygen extraction fraction (OEF) can be expressed using the effective diffusivity for oxygen in the capillary bed (D) as OEF = 1 - exp(-D/CBF). The D value is proportional to the microvessel blood volume. In this study, changes in D during neural activation and deactivation were estimated from changes in capillary and arteriole diameter measured by two-photon microscopy in awake mice. Capillary and arteriole vessel diameter in the somatosensory cortex and cerebellum were measured under neural activation (sensory stimulation) and neural deactivation [crossed cerebellar diaschisis (CCD)], respectively. Percentage changes in D during sensory stimulation and CCD were 10.3 +/- 7.3 and -17.5 +/- 5.3 % for capillary diameter of < 6 mu m, respectively. These values were closest to the percentage changes in D calculated from previously reported human positron emission tomography data. This may indicate that thinner capillaries might play the greatest role in oxygen transport from blood to brain tissue.