The relationship between cerebral blood flow autoregulation and oxygen tension of cerebral tissue is not fully known. We have examined the autoregulation of local cortical blood flow (CoBF) and cortical oxygen tension (bPO2) in the rabbit. CoBF was measured continuously by using the heated thermocouple technique and bPo2 was monitored by the polarographic method. Intravenous injection of phenylephrine hydrochloride or trimethaphan camsylate was used to test for autoregulation by increasing or decreasing perfusion pressure. The mean values of CoBF and bPO2 were 36 +/- 5 ml/100g/min and 32 +/- 12 mmHg respectively at 90 mmHg mean arterial blood pressure (MABP). The changes in both CoBF and bPO2 with changing perfusion pressure were often relatively small at near baseline blood pressure and became more pronounced with large increases or decreases in MAPB. On returning from high MABP to baseline blood pressure, a hysteresis effect on CoBF was observed in 20 out of 22 cases. The autoregulation of CoBF was maintained in the range between 80 and 100 mmHg MABP. On the other hand, bPO2 was maintained constant in the range between 75 and 110 mmHg. This range is significantly wider than that of CoBF. We conclude that autoregulation of CoBF limits changes in local CoBF to maintain constant oxygen tension in brain tissue.