The relative efficacy on the renal function of rabbits by oral administration of furosemide and 1 : 2 furosemide-PVP coprecipitate was compared by measuring the urine volume in response to maximal response and the amounts of electrolytes excreted in urine. The furosemide produced a rapid onset, short duration of diuresis, in contrast, the 1 : 2 furosemide-PVP coprecipitate, a rapid onset, significantly larger magnitude, and longer duration of diuresis and therefore the bioavailability of furosemide from the coprecipitate were increased significantly. The average urine volume and the amount of sodium and potassium excreted in urine were increased about 2.9,- 14.8-, and 1.8-fold from furosemide, and about 6.2-, 24.2-, and 3.6-fold from 1 : 2 furosemide-PVP 40,000 coprecipitate, respectively, comparing by their control values.