Excretory urography occasionally delineates striations of alternating density in the renal parenchyma. Experimental observations on a dog model of ureteric obstruction show that the dense striations have the same dynamic alterations as the obstructive nephrogram. They become progressively more dense to a plateau and then fade with relief of obstruction. Previous obstruction is shown to facilitate visualization of the striations presumably by having dilated the tubules. Previous obstructive episode, previous symptoms, or chronic obstruction were present in each of the clinical cases collected which show a similar striation pattern. We conclude that the dense striations represent shadows produced by hyperconcentrated contrast material which has accumulated in dilated collecting ducts or groups of ducts within the medullary rays.