✓ Recent morphological and functional studies on the circle of Willis suggest that the areas of supply of the six major cerebral arteries show a considerable variation in distribution, in contrast to the relatively consistent pattern generally accepted; therefore, the cortical and intracerebral distribution of the territories of these arteries was investigated in 25 unfixed human brains obtained at routine autopsy. The six major cerebral arteries were simultaneously injected under the same pressure with different-colored Araldite F mixtures under standardized conditions to obtain the most realistic territorial distribution. The cortical boundaries were examined and recorded in relation to the cerebral gyri and sulci, and the territories of the anterior, middle, and posterior cerebral arteries were analyzed and compared. The intracerebral distribution of these territories was investigated after the injected brains were cut in parallel slices. The variability of the territories of these arteries was much larger than generally described in the literature. Twenty-six variations in the territory of the anterior cerebral artery, 17 variations in the area of the middle cerebral artery, and 22 variations in the area of the posterior cerebral artery were found in the cortex of 50 hemispheres. Intracerebrally. the anterior, middle, and posterior cerebral arteries contributed in varying degrees to the blood supply of the lobar white matter, the internal capsule, the caudate nucleus, and the lentiform nucleus. The large variation in the area in which the cortical and intracerebral boundaries between these territories was located was demonstrated by illustrating the minimum and maximum extent of each. The results are compared with prior findings, and their implications for both experimental model studies and clinical practice are discussed.