1. The contribution of dietary constituents to the large urinary output of benzoic acid characteristic of ruminants and some herbivores is not well understood.
2. Methods for the analysis of quinic, cyclohexanecarboxylic, benzoic, phenylacetic, 3-phenylpropionic and cinnamic acids in urine and in rumen fluids were developed.
3. The urinary output of aromatic acids by sheep given seven rations was determined: benzoic acid output varied between 2·8 and 7·8 g/d; phenylacetic acid output between 0·16 and 1·3 g/d; cinnamic acid between 0·08 and 0·25 g/d and small amounts of 3-phenylpropionic acid were found in some samples.
4. Increments in urinary aromatic acid excretion were determined when the acids listed in paragraph 2 were infused via rumen or abomasal cannulas.
5. When cyclohexanecarboxylic acid was infused 40% of the dose was excreted as urinary benzoic acid after either route of infusion. Quinic acid was completely metabolized in the rumen: following rumen infusion between 16 and 53% of the infused acid was recovered as urinary benzoic acid; none was so recovered after abomasal infusion.
6. Urinary recoveries of rumen- and abomasally-infused aromatic acids were: benzoic acid 90 and 88% respectively as benzoic acid, phenylacetic acid 78 and 83% respectively as phenylacetic acid, 3-phenylpropionic acid 96 and 105% respectively as benzoic acid and cinnamic acid, 70 and 70% respectively as benzoic acid.
7. The concentration of aromatic acids in rumen fluid varied with time after feeding: cyclohexanecarboxylic acid was maximal (7 mg/l) 1 h after feeding, benzoic acid was always a minor component (0·5 ± 0·5 mg/l), phenylacetic acid varied between 0 and 35 mg/1 and 3-phenylpropionic acid between 25 and 47 mg/l. Cinnamic acid was not found in rumen fluid but on rumen infusion of this acid the concentration of 3-phenylpropionic acid in rumen fluid increased by 10 mg/l rumen fluid per g infused per d.
8. The incomplete metabolism of quinic and cyclohexanecarboxylic acids to urinary benzoic acid is discussed. It is concluded that the principal dietary precursors of urinary benzoic acid in ruminants are compounds yielding 3-phenylpropionic acid on microbial fermentation in the rumen. The small amount of cinnamic acid characteristic of ruminant urine arises as an intermediate in the β-oxidation of 3-phenylpropionic acid in the body tissues.