The influence of prazosin (0.1 ㎎/㎏ i. v.) on the excretion and diuretic action of furosemide (2㎎/㎏ i. v.) in rabbits was studied to investigate an interaction between α-adrenergic blocking agent, prazosin and furosemide. The results were as follows; 1) With the combined administration of prazosin and furosemide, the plasma concentration of furosemide was increased, the urinary excretion rate and renal clearance of furosemide were reduced, and the biological half-life of furosemide was increased. 2) The diuretic action of furosemide was significantly reduced with the combined administration of prazosin: maximal decrease in urine volume, urinary electrolytes, clearance of Na^+ and Cl^-, and GFR and RPF, as well as maximal increase in Na^+ reabsorption rate were noted 10 minutes after administration of furosemide (2㎎/㎏ i. v.)