Background and Purpose: We developed a model of acute focal ischemia in the territory of the anterior cerebral artery in baboons to study the ischemic pattern following occlusion and changes in regional cerebral blood flow. Methods: In nine anesthetized animals, a Scoville clip was placed on the proximal segment of the common anterior cerebral artery via a unilateral transorbital approach. Regional cerebral blood flow was measured by hydrogen clearance in the cortex and corpus callosum. Postexperimentally, arteries were selectively injected. Results: The resulting ischemia involved both hemispheres symmetrically and the corpus callosum. Cortical flows were significantly reduced within a region 15 mm from the midline on each side (p < 0.01). A gradient of cortical flow reduction was produced between 10 and 25 mm from the midline. This area defines the boundary region between the territories of the anterior and middle cerebral arteries, and is identified as the "penumbra" of the ischemic core, which itself lies within 10 mm of the midline. Blood flows in the corpus callosum decreased from an average of 21.0 to 6.7 ml/100 g/min in the body (p < 0.01) and from 22.5 to 10.7 ml/100 g/min in the genu (p < 0.05). Conclusions: This ischemic model has close physiological and morphological relevance to stroke-related clinical circumstances, in particular the acute conditions of focal cerebral ischemia associated with vascular surgery. It also provides a new framework for experimental investigation of the ischemic penumbra.