An intensively fluorescent product was formed in alkaline solution by hydrolysis of cephalexin at elevated temperature. The fluorescent product gave excitation and emission maxima at 355 and 435 nm, respectively. It was readily extracted into ethyl acetate and a acetone-chloroform mixture from an acidic solution and then extracted back into an alkaline solution. Using this new method, urinary excretion behaviors in human volunteers after oral administration of cephalexin capsule were investigated.