Oxygen pressure (pO2) in brain tumors, pO2 in brain cortex surrounding the tumors, and PaO2 were measured simultaneously during total resection in 16 patients with previously untreated brain tumors in order to detect hypoxic regions within the tumors. When the inhaled O2:N2O ratio was 1:3 under enflurane anesthesia, mean PaO2 was 109.2 +/- 5.8 mm Hg, a rather high value when compared with that obtained when air is inhaled under atmospheric pressure. The simultaneously measured intratumoral pO2 and pO2 in brain cortex surrounding the tumor were 15.3 +/- 2.3 and 59.8 +/- 6.5 mm Hg, respectively. Each intratumoral pO2 value was significantly lower than that of pO2 in brain cortex surrounding the tumor (mean less than 30 mm Hg, Wilcoxon signed rank test, p less than 0.005) and influenced the oxygen effects on radiation. These results appear to confirm that there are hypoxic regions within human brain tumors. A comparison between intratumoral pO2 and either the angiographic or contrast-enhanced computerized tomography scans of the tumor vasculature disclosed no correlation.