The gyral topography of the ischaemic penumbra associated with middle cerebral artery occlusion was studied in cats; local blood flow (hydrogen clearance, 2-min initial slope analysis), pial surface potassium activity (Kp), and electroencephalogram (EEG) amplitude were recorded on the ectosylvian, suprasylvian, and marginal gyri. Penumbral conditions were defined as a reduction of EEG amplitude in the absence of a major increase in Kp. Whole hemisphere cerebral blood flow prior to occlusion was 35.6 +/- 11.4 (SD) ml 100 g-1 min-1 (n = 25), and fell significantly (p less than 0.001) to 13.0 +/- 4.7 (SD), 14.1 +/- 6.6, and 23.8 +/- 9.3 on ectosylvian, suprasylvian, and marginal gyri, respectively. Pre-occlusion Kp was 3.0 +/- 0.9 mM (n = 53); sustained, steady-state increases in excess of 11.5 mM occurred in 5 of 7 (71%) experiments on ectosylvian gyrus, in 13 of 22 (59%) experiments on suprasylvian gyrus, and in 2 of 24 experiments on marginal gyrus. Transient increases in Kp occurred at occlusion and resolved to baselines significantly higher than control in 5 (23%) experiments in suprasylvian gyrus and in 15 (63%) on marginal gyrus. In 7 experiments on the marginal gyrus (29%), no increase in Kp greater than 1 mM occurred. In contrast, EEG amplitude on this gyrus was reduced by middle cerebral artery occlusion in 25 of 26 experiments, and it is concluded that this gyrus is the principal site of ischaemic penumbra in the model; in 41% of experiments similar conditions were also present on the suprasylvian gyrus.