In this communication we review selected experiments involving the use ofcarbon paste electrodes (CPEs) to monitor and measure brain tissue O2 levels in awakefreely-moving animals. Simultaneous measurements of rCBF were performed using the H2clearance technique. Voltammetric techniques used include both differential pulse (O2) andconstant potential amperometry (rCBF). Mild hypoxia and hyperoxia produced rapidchanges (decrease and increase respectively) in the in vivo O2 signal. Neuronal activation(tail pinch and stimulated grooming) produced similar increases in both O2 and rCBFindicating that CPE O2 currents provide an index of increases in rCBF when such increasesexceed O2 utilization. Saline injection produced a transient increase in the O2 signal whilechloral hydrate produced slower more long-lasting changes that accompanied the behavioralchanges associated with anaesthesia. Acetazolamide increased O2 levels through an increasein rCBF.