One g. of urocanic acid a (m. 218[degree]) is dissolved on the H2O bath in 60 cc. 80% alcohol, cooled to room temp., and the soln. exposed in a 100 cc. quartz flask to a Hanauer analytical lamp at a distance of 20 cm. for 8 hrs. During the exposure a stream of N is passed through the flask. The soln. is heated with animal charcoal, and the filtrate concd. on the H2O bath. The first crystals obtained are unchanged urocanic acid a. From the mother liquor, broom shaped clusters of needles, urocanic acid b (C6H6N2O2, m. 175[degree]-176[degree]) are obtained. This acid is isomeric with urocanic acid a. The nitrate of urocanic acid a melts at 190[degree], and the picrolonate at 268[degree]; the nitrate of urocanic acid b melts at 156[degree], and the picrolonate, at 243[degree]. It is believed that the a acid is the trans form, and the b acid, the cis form. Two ml. of M/10 neutralized urocanic acid b, 5 ml. urocanase extract from rat liver, and M/15 phosphate buffer up to 20 ml. are incubated for 24 hrs. at 38[degree]. The b acid is not attacked by urocanase and is probably the unnatural form, while the a acid is probably a natural intermediate product present in the organism.