The mammalian degradation of lysine is believed to proceed via two distinct routes, the saccharopine and the pipecolic acid routes, that ultimately converge at the level of α-aminoadipic semialdehyde (α-AASA). α-AASA dehydrogenase-deficient fibroblasts were grown in cell culture medium supplemented with either l-[α-¹⁵N]lysine or l-[ε-¹⁵N]lysine to explore the exact route of lysine degradation. l-[α-¹⁵N]lysine was catabolised into [¹⁵N]saccharopine, [¹⁵N]α-AASA, [¹⁵N]Δ¹-piperdeine-6-carboxylate, [¹⁵N]α-AAA, and surprisingly in [¹⁵N]pipecolic acid, whereas l-[ε-¹⁵N]lysine resulted only in the formation of [¹⁵N]saccharopine. These results imply that lysine is exclusively degraded in fibroblasts via the saccharopine branch, and pipecolic acid originates from an alternative precursor. We hypothesize that pipecolic acid derives from Δ¹-piperdeine-6-carboxylate by the action of Δ¹-pyrroline-5-carboxylic acid reductase, an enzyme involved in proline metabolism.