We have examined how various challenges to brain acid-base homeostasis, resulting in extracellular acidosis, after N-methyl-D-aspartate (NMDA)-evoked depolarizations in vivo. Repeated stimuli were produced by perfusion of 200 μM NMDA for 2 min through a microdialysis probe implanted into the striatum of halothane anesthetized rats. Hypercapnia reduced NMDA-evoked responses in a concentration-dependent manner, with 7.5 and 15 % CO 2 in the breathing mixture reducing the depolarization amplitude to 74 % and 64 % of that of the initial stimuli, respectively. Application of 50 mM NH 4 + progressively reduced dialysate pH, and a further acidification was observed when NH 4 + was discontinued. Perfusion of NMDA after NH 4 + application evoked smaller depolarizations (56 % of the corresponding control, 5 min after NH 4 + removal), and this effect persisted for over 1 h. Perfusion of acidic ACSF did not alter the amplitude of NMDA-evoked depolarization, despite changes in dialysate pH confirming that exchange/buffering of acid equivalents took place between the perfusion medium and the surrounding tissue. This negative result probably reflected the remarkable capacity of the brain to buffer H + . Together, these results demonstrate that extracellular acidosis, such as that associated with excessive neuronal activation or ischemia, inhibits NMDA-evoked responses in vivo.