L-pipecolic acid and fluoresced cherry red under ultraviolet light as reported by Morrison.2 An assay of the pipecolic acid hydrochloride showed that it had a specific activity of 1.21 X lo5 disintegrations/min./mmole. Part of the pipecolic acid hydrochloride was converted to the hydantoin4 and this derivative had a specific activity of 1.23 X lo5 disintegrations/min./mmole. These observations afford strong evidence that pipecolic acid is a catabolite of L-lysine in the rat.The high specific activity obtained suggests that pipecolic acid is involved in the conversion of L-lysine to a-aminoadipic acid, a view in keeping with the finding that under similar experimental conditions, L-lysine, via a-aminoadipic acid, yields glutaric acid with a specific activity of 6.45 X lo4 disintegrations/min./mmole.6