We previously reported (J Appl Physiol 89: 807–822, 2000) that ≤10 min of hyperbaric oxygen (HBO2; ≤2,468 Torr) stimulates solitary complex neurons. To better define the hyperoxic stimulus, we measured PO 2 in the solitary complex of 300-μm-thick rat medullary slices, using polarographic carbon fiber microelectrodes, during perfusion with media having PO 2 values ranging from 156 to 2,468 Torr. Under control conditions, slices equilibrated with 95% O2at barometric pressure of 1 atmospheres absolute had minimum PO 2 values at their centers (291 ± 20 Torr) that were ∼10-fold greater than PO 2values measured in the intact central nervous system (10–34 Torr). During HBO2, PO 2 increased at the center of the slice from 616 ± 16 to 1,517 ± 15 Torr. Tissue oxygen consumption tended to decrease at medium PO 2 ≥ 1,675 Torr to levels not different from values measured at PO 2 found in all media in metabolically poisoned slices (2-deoxy-D-glucose and antimycin A). We conclude that control medium used in most brain slice studies is hyperoxic at normobaric pressure. During HBO2, slice PO 2 increases to levels that appear to reduce metabolism.