Our objective is to investigate the properties of the renal circulation and the role it plays in nephron functions. We are particularly interested in the effects of changes in the medullary circulation on urine concentrating mechanisms and in the role of distribution of blood flow to different nephrons in controlling sodium excretion rate and urine concentrating mechanisms. We intend to quantitate the peak transit times of a filterable dye through cortical and medullary structures. The methods to be used will permit quantitation of transit times through the renal artery, renal vein, cortical surface capillaries, proximal and distal convoluted tubule, vasa recta, long loops of Henle and collecting ducts. These measurements permit assessment of several parameters of nephron function as well as changes in the circulating system.