To investigate the dynamics of tissue oxygen demand and supply during brain functions, partial pressure of oxygen (Po2) and local cerebral blood flow (LCBF) were simultaneously recorded with an oxygen microelectrode and laser Doppler flowmetry, respectively, in rat somatosensory cortex. One, two, and five sec long electrical hindlimb stimuli were applied to vary the duration of evoked cerebral metabolic rate of oxygen (CMRO2). The electrical stimulation induced a robust increase in Po2 (4 - 9 torr at peak) following an increase in LCBF (14 - 26% at peak). A consistent lag of ~1.2 sec (0.6 - 2.3 sec for individual animals) in the Po2 relative to the LCBF was found irrespective of stimulus length. It is argued that the lag in Po2 was predominantly caused by the time it takes for oxygen to diffuse through tissue. During brain functions, the supply of fresh oxygen further lagged due to the latency of the LCBF onset (~0.4 sec). The results indicate that the tissue oxygen supports excess demand until the arrival of fresh oxygen. However, a large drop in Po2 was not remarkably observed, indicating that the evoked neural activity demands little extra oxygen, or that the time-course of excess demand is as slow as the increase in supply. Thus, the dynamics of Po2 during brain functions predominantly depends on the time-course of LCBF. Possible factors influencing the lag between demand and supply are discussed, including vascular spacing, reactivity of the vessels and the diffusivity of oxygen.