We measured the relationships between changes in extracellular pH (pHe), potassium (Ke), and calcium (Cae) activities and DC potential (DCe) in progressive ischaemia of rat cerebral cortex. pHe and Ke, or Cae and Ke, were measured at the same point simultaneously, using triple-barrelled, double-ion-sensitive microelectrodes. Ischaemia was produced using bilateral carotid artery occlusion and hypotension in rats under 50% N2O-0.4% halothane anaesthesia. Unilateral carotid artery occlusion did not affect blood flow, but bilateral occlusion reduced flow to approximately 40% of normal. Autoregulation of blood pressure (BP) changes was lost after bilateral occlusion, and so progressive hypotension produced a linear decrease in flow. pHe began to decrease at high levels of flow (30-35 ml 100 g-1 min-1) and showed stepwise acidotic shifts with reductions in BP. Ke was affected at flows of approximately 15 ml 100 g-1 min-1, during which time it was critically dependent on BP. When Ke reached 6 mM, it increased rapidly to 40 mM and was associated with a negative shift in DCe. When Ke reached approximately 10 mM, Cae decreased rapidly to approximately 0.1 mM. pHe had reached 6.87 when Ke increased rapidly and showed a transient alkalotic shift of approximately 0.14 units at that time. Possible mechanisms for the sequence of ion changes described are discussed.