Abstract Four assay methods were tested for the measurement of Δ 1-piperideine-2-carboxylate, a proposed alicyclic ketimino acid intermediate in the pathway of lysine metabolism to l-pipecolate, and the product of d-amino acid oxidase on d-pipecolate. The method using Δ 1-piperideine-2-carboxylate reductase from Pseudomonas putida was found to be most sensitive and specific. Measurement of Δ 1-piperideine-2-carboxylate by reduction with NaBH 4 and ninhydrin assay of the resultant pipecolate, by direct acidic ninhydrin assay, and by o-aminobenz-aldehyde assay were less desirable because of lower sensitivity and specificity. Two synthetic methods for preparing l-[ 14C]pipecolate from the racemic dl-[ 14C]pipecolate were investigated. Incubation of dl-[ 14C]pipecolate with a combination of d-amino acid oxidase and Δ 1-piperideine-2-carboxylate reductase or d-amino acid oxidase and NaBH 4 totally inverted the d-isomer to the l-isomer, with Δ 1-[ 14C]piperideine-2-carboxylate as an intermediate in each cycle of interconversion. No purification except desalting through a Dowex 50 (H +) column was necessary in order to recover l-[ 14C]pipecolate in pure form. The yield was 95–97% compared to <50% in the conventional method.