Changes in extracellular ion activities were measured during partial ischaemia of the cerebral cortex of primates anaesthetized with alpha-chloralose. Triple-barrelled, double-ion-sensitive microelectrodes were used to measure the extracellular potassium (Kc) and calcium (Cac) activity at the same point simultaneously. The ion changes were related to local cerebral blood flow, and it was shown that at a blood flow of approximately 10 ml 100 g-1 min-1, there is a threshold below which ion homeostasis is disturbed. This is associated with a dramatic rise in Ke and fall in Cae. Cae falls from a normal value of 1.31 +/- 0.1 mM to approximately 0.28 mM in densely ischaemic tissue. In ischaemia, Ke reaches 13.4 +/- 3.8 mM before Cae begins to fall. The fall in Cae, although related to reduced blood flow, is closely associated with and follows the rise in Ke. The change in Cae is probably due to an increase in membrane permeability, as a result of either depolarisation or a critical lowering of cellular energy reserves.