A sensitive method using the Technicon Amino Acid Autoanalyzer was developed for the determination of l-tyrosine O-sulphate in mammalian urine. Normal rabbit urine was shown to contain a material that has properties identical with l-tyrosine O-sulphate. The material is also a normal constituent of rat, sheep, calf and mouse urine and is excreted by these animals in quantities ranging from 0.1 to 0.8mg./day/kg. body wt.

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2. The isomers in the major dienoic acid fraction had c8-c8 non-conjugated configurations, and the double bonds were largely at C-11 or C-12 and C-15 or C-16. 3. The monoenoic acids formed were largely trans, with the double bond predominantly at C-13 or C-14. 4. Rapid hydrogenation of [1-14C]linoleic acid and [1-14C]oleic acid occurred in the artificial rumen. The former gave a trans monoenoic acid as an intermediary. 5. The tran8 C18 monoenoic acids passing from the rumen were almost quantitatively absorbed in the ileum. 6. The unsaturated C18 acids present in the ileum digesta were hydrogenated in the caecum and colon, so that nearly all the acids in the excreta were saturated. 7. The hydrogenation of [1-'4C]linoleic acid by caecal and colon contents resulted again in the formation of substantial amounts of trans C18 monoenoic acids. T. W. S. was in receipt of a Fellowship from the International Atomic Energy Agency during the course of this work. The Agency is also thanked for a grant towards the purchase of radioactive substrates and measuring equipment. Mr N. Huskisson and Mr N. Clarke are thanked for their valuable assistance.