The effects of furosemide on the renal concentrating mechanism and on intrarenal hemodynamics were assessed in isolated blood-perfused dog kidneys. The natriuretic, diuretic and hemodynamic responses to furosemide were lesser in magnitude and shorter in duration than those seen in intact animals. Indocyanine green dye dilution curves were used to assess the intrarenal distribution of blood flow. The washout curve prior to furosemide was described by three components. After furosemide, the curve was described by four components. The greatest change in intrarenal distribution of blood flow after furosemide occurred in the noncortical compartments. The data suggest a disproportionately greater increase in medullary blood flow after furosemide. Blood flow through the slow-flow compartments increased 107% whereas flow through the fast-flow compartment increased only 7%. This is in agreement with our previous hypothesis that the increase in renal blood flow and, more importantly, the redistribution of blood flow, produced by furosemide can markedly enhance the effect of the drug, particularly on the concentrating mechanism.