Determination of glucuronic acid was found to be possible by mixing 0.2% naphthoresorcinol reagent, 6N hydrochloric acid, and sample solution, heating this mixture in a boiling water bath, extracting the dye formed with benzyl alcohol, and measuring absorbancy of the extract solution at 615mμ. The determinable range was 5-125γ of the acid per 2cc. of solution. This method was found to be applicable for following variation of the amount of glucuronic acid in a definite amount of glucose, urine, or blood.