Recently, a model of cerebral oxygen delivery was described (J Appl Physiol 85:554) which yields a relationship similar to that used to depict substrate transport across the endothelium. Because the endothelium is not a diffusion barrier for oxygen, the permeability surface area product was replaced by an effective mass transfer coefficient term for oxygen, D. The cerebral metabolic rate of oxygen utilization (CMRO2) was linked to cerebral blood flow (CBF) and volume (CBV) through properties that modify the vessel-to-tissue oxygen tension giving rise to changes in D. Changes in the value of D were correlated with changes in CBF, CMRO2, and CBV as measured using NMR methods in a 48 microL volume of the cerebral cortex of anesthetized rats at different levels of activity. We conclude that the changes in total vascular volume (i.e., swelling or shrinking of the capillary bed) contributes < 5% to changes in D, whereas variations in the number of hematic vs. plasmatic capillaries, or intra-capillary stacking vs. unpacking of erythrocytes, or increase vs. decrease of dissolved oxygen in the tissue (i.e., processes which modify vessel-to-tissue oxygen tension) contribute(s) > 95% to changes in D.