We compared the difference in brain tissue oxygen pressure (pO2), carbon dioxide pressure (pCO2), pH and temperature with 2 probes inserted 1 cm apart, in 7 patients. Following a craniotomy for cerebrovascular surgery, two Neurotrend probes which measure pO2, pCO2, pH and temperature were inserted into the brain 1 cm apart. Measures were compared between the probes under baseline anesthetic conditions and during the course of surgery. Under baseline conditions, tissue pO2, pCO2, pH and temperature were not different between the 2 probes. A significant correlation was seen between the probes in pH and temperature. During the course of surgery, variation in tissue gases and pH occurred with changes in ventilation and blood pressure but the difference between the probes remained stable. Ischemic changes in pO2, pCO2 and pH were seen in one of the 2 probes during brain artery occlusion or retractor placement. These results show that tissue pO2, pCO2 and pH are consistent in local brain regions during steady state conditions. The relationship between local measures is disrupted by regional ischemia.