Absorption, distribution, excretion, and metabolism of 5,8-dihydro-5-methoxy-8-oxo-2H-1, 3-dioxolo[4,5-g]quinoline-7-carboxylic acid(miloxacin), a new antimicrobial agent, were studied in female rats by using 14C-miloxacin which was administered orally to the animals in a dose of 50 mg/kg. 14C-Miloxacin was absorbed rather fast and the radioactivity of 14C distributed widely in a variety of tissues. Peak concentrations of 14C in serum and tissues occurred 1 to 2 hr after dosing, and were approximately 60 micrograms equivalent of miloxacin per ml or g in serum, liver and kidney. Excretion of 14C in urine and feces was fast, and recoveries of 14C during 48 hr period were approximately 30% in urine and 60% in feces. Concentrations of intact 14C-miloxacin were higher in serum and kidney while lower in liver. Major metabolites in excreta were the demethoxy derivative (M-1) and the glucuronide of miloxacin; and as minor metabolites five other metabolites were identified. As sex differences, the following facts were observed; concentrations of 14C in serum and tissues were generally 1.2 to 1.6 times higher in female rats those in male rats, and the capacity to metabolize miloxacin, especially in the glucuronic acid conjugation, was rather lower in female rats than that in male rats.