Extracellular pH and P-O2 was recorded in the isolated in vitro brainstem of the metamorphic tadpole, Rana catesbeiana while the brainstem preparation was superfused with oxygenated mock cerebrospinal fluid of pH = 7.8, P-CO2 = 17 Torr, P-O2 = 600 Torr at 23 degrees C. Using pH and P-O2 microelectrodes, the ventral medullary surface was penetrated at midline and lateral sites between cranial nerves V and X. Mean pH and P-O2 gradients of 0.07 pH units/100 mu m and 60 Torr/100 mu m were detected in the superfusate, 100-200 mu m above the ventral surface of the brainstem. These gradients remained virtually constant for the first 100-200 mu m below the medullary surface. Beyond this level, pH and P-O2 gradients decreased in a curvilinear fashion. For midline tracts, minimum values of pH and P-O2 (7.58 +/- 0.05 and 323 +/- 31 Torr) were reached at a depth of 500-750 mu m, whereas for lateral tracts, mean minimum values of pH and P-O2 (7.34 +/- 0.12 and 240 +/- 68 Torr), were recorded at 850-900 mu m. With further electrode advancement, pH and P-O2 gradients in both midline and lateral tracts reversed as levels began to increase. Between CN V and X, lateral width was 4.34 +/- 0.57 mm, while dorsal-ventral thickness in midline and lateral regions was 0.92 +/- 0.21 and 1.31 +/- 0.22 mm, respectively. Overall, the in vitro tadpole brainstem provides a robust neural preparation which, although moderately acidic, is well oxygenated throughout all tissue layers. (C) 1997 Elsevier Science B.V.