The blood-brain barrier is believed to be relatively impermeable to changes in arterial blood H+ and OH- concentrations. Brain surface pH was measured during 30 min of isocapnic metabolic acidosis or alkalosis induced by i.v. infusion of 0.2 N HCl or NaOH in anesthetized dogs. The mean brain surface pH fell significantly by 0.06 and rose by 0.04 pH units during HCl or NaOH infusion, respectively. Respective changes were observed in the calculated cerebral interstitial fluid [HCO3-]. There were no significant changes in cisternal CSF acid-base variables. Changes in arterial blood H+ and OH- concentrations are apparently reflected in brain surface pH relatively quickly. Such changes may contribute to acute respiratory adaptations in metabolic acidosis and alkalosis.