The effect of the lysosomal contents of hog kidney cortex, especially of the fraction not bound by concanavalin A (Fraction A) on the permeability of the coronary and cerebral arteries of rats was studied ultrastructurally using H.R. peroxidase. This fraction was devoid of renin activity by bioassay. The coronary arteries of the experimental rats displayed fibrinoid degeneration: e.g., degeneration and disappearance of medial smooth muscle cells and deposition of electron dense materials containing fibrin. A large amount of reaction product of peroxidase was present in the subendothelial space and media where fibrinoid degeneration was evident. Transendothelial passage of the marker occurred by both junctional and vesicular transport. There was no evidence of separation or discontinuity of the endothelial cells. Occasionally, increased permeability of the intima was noted in the coronary arteries without medial damage. By contrast, neither fibrinoid degeneration nor increased permeability was noted in the cerebral arteries. The difference in the response of the two arteries seems attributable to the barrier effect of cerebral arterial intima.