Heterogeneity of microregional O2 supply and consumption balance exists in the brain. Previous studies have shown that anesthetics such as pentobarbital and isoflurane, decrease the heterogeneity of O2 saturation in the small veins of the brain. This study was performed to determine whether ketamine, an anesthetic, would alter this heterogeneity. Rats were anesthetized with isoflurane for the cannulation of the femoral artery and vein. Ninety minutes after discontinuation of isoflurane, they either received injections with ketamine 100 mg/kg intraperitoneally (IP) (n = 7) or were used as conscious controls (n = 7). In each rat, regional cerebral blood flow (rCBF) was determined by 14C-iodoantipyrine autoradiography, and microregional arterial and venous (diameter 20-100 microns) O2 saturation was measured by microspectrophotometry in the anterior cortex, posterior cortex, and pons. The average rCBF and O2 consumption were similar between the two groups in each brain region except in the anterior cortex, where the O2 consumption of the ketamine group was slightly higher than that of the conscious animals. The average regional O2 supply-to-consumption ratios were similar in the three brain regions in both of the groups. However, there was heterogeneity of the O2 saturation in small veins in all the brain regions that we studied. The coefficient of variation [CV = (SD/mean X 100] of venous O2 saturation of the ketamine group in the anterior cortex (19.8 +/- 7.2), in the posterior cortex (18.8 +/- 9.4), and in the pons (16.7 +/- 6.5) was not significantly different from that in the conscious animals (22.8 +/- 6.4, 23.1 +/- 5.3, and 15.7 +/- 6.4, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)