The possible use of impedance measurement with scalp electrodes to detect cerebral ischemia non-invasively was investigated in the anaesthetised rat. Global cerebral ischaemia was induced by diathermy of both vertebral arteries and reversible occlusion of the carotid arteries. Impedance was measured at 50 kHz by a four electrode method. With cortical electrodes in various positions, reversible impedance increases of 15 - 60% were recorded during episodes of cerebral ischaemia which lasted for 5 - 30 min. With electrodes placed in overlying positions in the scalp, impedance increases had a similar time course but their amplitudes were about 10 - 20% of the increases measured on the cortex. These were not due solely to changes in temperature which accompanied cerebral ischaemia or local changes in the scalp. These findings suggest that it may be possible to image cerebral ischaemia in human subjects non-invasively with Electrical Impedance Tomography and scalp electrodes.