The influence of penicillin G infusion(7.5 units/min/kg) on the diuretic action of furosemide(250μg/kg) in rabbits was studied to investigate the drug interaction between penicillin G and furosemide. The results were as follows: 1) There was no significant change in renal function in the penicillin G infused rabbits compared with the normal ratbit. 2) In comparison with the control group, a significant increase in △U_flow, △U_NaV, △U_kV and△U_clV was noted by 30 minutes following the administration of furosemide in the penicillin G infused group. There was no significant difference in △C_In, △C_PAH between the two groups and △Na⁺ reabsorption rate was significantly decreased in the latter. The potentiating effect of penicillin G on the diuretic action of furosemide should be due to the increased luminal concentration of free form of furosemide, and it may be related to the competitive inhibition of plasma protein binding between the two drugs.