The morphogenesis of massive cerebral hemorrhagic lesions was studied in 6 experimental hypertensive rats. Hemorrhage due to rupture of arteries caused by hypertensive arterial lesions (plasmatic arterionecrosis) was found in the middle cerebral arteries in the subarachnoid space and immediately after their penetrating into the cerebral parenchyma. This primary hemorrhage induced, through circulatory disturbances, secondary bleedings of small vessels and veins, which were all combined to give rise to a massive hemorrhagic lesion. The hypertensive arterial lesions, which were the direct cause of the cerebral hemorrhage, were constituted by necrosis and disappearance of smooth muscle cells and blood plasma infiltration in the media, blood plasma infiltration, especially deposition of fibrin, having periodic cross striation of about 200 Å (fibrinoid degeneration) in the intima, and dissolution and disappearance of the internal elastic lamina