Creatine is found in the urine of subjects ingesting creatine monohydrate as an ergogenic aid. Creatinine, the catabolic breakdown product of creatine, is a major constituent of normal urine. It is of interest to follow the excretion of creatine and creatinine in urine as a function of time after creatine ingestion. In this study, creatine and creatinine were analyzed in urine by capillary electrophoresis. The optimization of the method was discussed, with the best results being obtained using a 30 mM phosphate-150 mM sodium dodecyl sulfate buffer at pH 6, with the detector set at 214 nm and an applied voltage of 15 kV across a 45 cm capillary. Verification of the method was provided by HPLC analysis and spiking. The application of the method was demonstrated by analysis of creatine and creatinine in urine samples collected in a 24-h period following creatine ingestion.