Effects of fosfomycin-sodium (FOM-Na) upon renal functions were studied with male rats. 1) After fasting for 24 hours, 25 ml/kg body weight of physiological saline were orally administered and respectively 160 and 320 mg/kg of FOM-Na were intraperitoneally administered immediately thereafter. The subsequent urinations amounts at 4 hours' interval were determined to see no effect of FOM-Na upon the urination amount. 2) After suspending water supply for 2 days, 1,000 mg/kg of FOM-Na were intraperitoneally administered once a day for successive 2 days. On the 3rd day, 50 ml/kg of dextran were intraperitoneally administered, and one hour thereafter, 1,000 mg/kg of FOM-Na were intramuscularly administered to hind leg. Blood samples were collected the following day (suspension of water was continued until blood sample collection) to determine the BUN and UA in serum. FOM-Na was found exerting on reinforcing effect upon renal dysfunctions caused by dextran.