Taurine is made to react with o-phthalaldehyde in the presence of urea and phosphate ions, and on acidifying the mixture with acetic acid a purple product is formed with an extinction maximum at 560 nm. A method based on this reaction is described for the quantitative determination of taurine and it is applied to the determination of this amino-acid in rat brain after passage of the tissue extract through ion-exchange resins. The effect of other amino-acids on the accuracy of the method is discussed.