Ten rabbits with transparent ear chambers were grafted with small pieces of kidney. The resulting vessel anastomosis and restoration of renal blood flow allowed continuous microscopic observations of functioning renal tissue in vivo. When the grafts were well established, acute renal failure was induced in the rabbit by glycerol injection. All cases showed similar changes. Within minutes the brisk blood flow within the renal grafts became progressively more sluggish until complete stasis was established. The initial change was a blanching of intertubular and glomerular capillaries with progressive dilation and stasis of renal veins. Only after almost complete cessation of blood flow in most graft vessels, generally after a further 10 minutes, was any change in arteries or arterioles observed. The afferent arterioles and then larger arteries showed constriction followed by complete stasis. The ear chamber vessels (nonrenal) continued to flow normally. Blood flow was slowly re-established in the grafts and by the next day was normal in the surviving rabbits. These studies provided visual in vivo evidence that the mechanism of glycerol-induced acute renal failure is mediated by a reversible renal ischemia and that the factors responsible act particularly on renal vasculature. However, the mechanism whereby blood flow ceases is obscure and it cannot be attributed to arterial or arteriolar constriction.