In 8 gerbils (Meriones unguiculatus) focal cerebral ischemia was produced by occlusion of the left common carotid artery and the opposite external carotid artery. After two hours blood flow was measured with iodoantipyrine labeled with carbon 14, and evaluated by means of quantitative autoradiography. Thereafter the same brain sections were stained for regional potassium by means of a histochemical technique. Changes in tissue potassium content were assessed by measuring the differences in optical densities in homotopic brain regions of the stained sections. The correlation between blood flow and tissue potassium level revealed that below a flow threshold of about 0.23 ml/gm/min, a definite potassium loss from the tissue was observed. The combination of autoradiographic methods with a technique for measuring the regional distribution of potassium may be useful in providing additional information on the occurrence of disturbed electrolyte homeostasis after the onset of focal ischemia.