CHANGES IN VENTILATORY rate affect arterial blood pH and PCO2within seconds to minutes, but the corresponding acute changes for cerebrospinal fluid (CSF) pH and PCO2have not been as well documented. Using our previously-described swine model of brain retraction ischemia, we examined changes in arterial and CSF pH and PCO2with acute changes in ventilation in four animals. Newly developed fluorescent dye technology permitted near-instantaneous recording of CSF pH and PCO2during acute hyperventilation (end-tidal PCO2of 20 mm Hg) and acute hypoventilation (end-tidal PCO2of 50 mm Hg). The Puritan-Bennett 3300 Intra-Arterial Blood Gas Monitor (PB3300) was used with the sensor placed in the CSF in the interhemispheric fissure posterior to the corpus callosum. The following data were gathered at 5, 15, 30, and 60 minutes after the ventilatory change: arterial pH and PCO2, end-tidal CO2, laser-Doppler cerebral blood flow, and CSF pH and PCO2. The baseline (normoventilation) values for arterial and CSF pH and PCO2in swine were comparable to those in humans: arterial pH 7.44 and PCO243 mm Hg; CSF pH 7.31 and PCO255 mm Hg. Changes in pH and PCO2with hyperventilation and hypoventilation occurred rapidly in both arterial blood and CSF. Steady-state values were reached within 15 minutes for hypoventilation, and 30 minutes for hyperventilation. The correlation between arterial and CSF values for both pH and PCO2at 5, 15, 30, and 60 minutes were all very highly significant (P< 0.001) except for arterial and CSF PCO2at 5 minutes (P< 0.01). CSF pH and PCO2follow arterial blood gas pH and PCO2rapidly with acute changes in ventilation. These results are consistent with previous research, and indicate the PB3300 may be useful in monitoring CSF pH and PCO2.