O 2 concentration ([O 2 ]) in air equilibrated solutions at room temperature is three fold higher than that in brain extracellular fluid (ECF), and CO 2 concentration ([CO 2 ]) is 100 times lower. Using microdialysis the ECF is routinely dialyzed against glucose free isotonic perfusates containing 200 μM O 2 and 10 μM CO 2 . In conscious rats, 2 days after probe implantation, decreasing perfusate [O 2 ] from 200 to 68 μM (physiologic level) for 60 min, while maintaining a low [CO 2 ] (10 μM), increased striatal dialysate glucose and lactate by 12% and 33%, respectively. The same protocol on the third day essentially had no effect on monoamine metabolites. Decreasing [O 2 ] concurrent with increasing [CO 2 ] to 1.3 mM (physiologic level) for 60 min increased glucose and lactate by 17% and 37%, respectively. This study demonstrates for dialysis studies of glucose and lactate, perfusates that mimic physiologic ECF [O 2 ] and [CO 2 ] are more appropriate.