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‐piperideine‐6‐carboxylate, 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‐piperideine‐6‐carboxylate by the action of Δ1‐pyrroline‐5‐carboxylic acid reductase, an enzyme involved in proline metabolism.