An on-line pH meter that can be mounted in microdialysis systems is described. The pH monitoring system was tested in rat cortex before and after middle cerebral artery occlusion (focal ischemia model). After probe implantation, pH values in the dialysate quickly reached a stable level that depended on perfusion medium (6.72, Ringer; 6.47, 0.9% saline) and flow rate (2 microliters/min). During ischemia, pH values sank rapidly and significantly, whereas lactic and ascorbic acid levels in the dialysate increased 9- to 12-fold. The pH of the dialysate is lower than that of the extracellular fluid because the relative recovery of carbon dioxide is about twice that of bicarbonate at the flow rate used, as shown in in vitro experiments. The pH meter would provide useful additional information during monitoring for ischemia, not only in experimental situations but also during neurosurgical intensive care. In the latter case, the on-line pH value would be a bedside parameter enabling fast feedback for setting analytical priorities and making therapeutical decisions.