Four different methods were evaluated for the determination of the creatinine content of a series of 24-hour urine samples. The results indicate that the methods of Folin, Hare, and Van Pilsum give essentially the same values. The precision of the method of Sullivan and Irreverre was extremely poor and the values obtained were 2-8 times higher than those obtained by the other technics. The compound, 1,4-naphthoquinone, used in the procedure of Sullivan and Irreverre, was synthesized and compared with a commercial sample. Data concerning 1,4-naphthoquinone-2-potassium sulfonate is given to clarify its identity.