- To study the effect of renal ischemia on the circulation in the inner medulla, blood flow in descending and ascending vasa recta was determined by fluorescence videomicroscopy in the exposed papilla of the uninephrectomized rat after clamping of the renal artery for 45 min. Total renal blood flow was determined in parallel studies with an electromagnetic flowmeter. Animals were studied 90 min (group 1E) and 24 h (group 2E) after right nephrectomy and release of the left renal artery clamp. Control rats were studied 90 min (group 1C) and 24 h (group 2C) after right nephrectomy alone. In groups 1E and 2E, total renal blood flow was reduced to 70 and 80% of that in their respective controls; renal vascular resistance increased by 50 and 73%, respectively. In striking contrast, blood flow was markedly elevated in descending and ascending vasa recta in groups 1E and 2E compared with the values in their respective uninephrectomized controls. These results indicate that the circulation in the inner medulla is rapidly restored after 45 min of total renal ischemia and that vasa recta blood flow rises above normal after 90 min and 24 h, despite a reduction in total renal blood flow and an increase in renal vascular resistance.