Air embolization of the brain produces cerebral ischemia that can be focal and reversible. The method has previously been hampered by (1) lack of selective arterial injection of the embolus, (2) disruption of local hemodynamic relationships by ligation of major arterial channels, (3) excessive volume of the air embolus, and (4) uncontrolled bubble size. To minimize these factors, a technique was devised in the rat whereby a fine catheter was advanced through a branch of the external carotid artery into the internal carotid artery. Air emboli of 5 microliters were found to arrest blood flow and to attenuate electrical activity in the ipsilateral cerebral hemisphere for seconds to a couple of minutes. The duration of ischemia varied from region to region, and it tended to be prolonged by arterial hypotension. In the nonembolized hemisphere, CBF never declined abruptly (indicating no cross-over of air) although electrical activity was suppressed in two-thirds of the animals.