We used angiographic and microsphere methods to evaluate the anatomic and functional features of renal collateral circulation in the rat. By the microsphere method, renal parenchymal blood flow was less than 1% of control 1 hour after occlusion of the main renal artery; 2.8% of control 1-2 weeks after arterial occlusion; and 1% of control 4-9 weeks after occlusion. Radiographic observations during chronic occlusion revealed numerous collateral vessels to the kidney. These vessels readily filled with angiographic contrast medium but the intrarenal circulation did not visualize. We conclude that collateral circulation to renal parenchyma is negligible after acute or chronic occlusion of the main renal artery in the rat. The rich anatomic plexus of collateral vessels has no functional significance and is unable to preserve viability of the parenchyma.