Acetyl-L-carnitine-d is intended for use as an internal standard for the quantification of L-acetylcarnitine by GC- or LC-MS. Acetyl-L-carnitine is an acetylated form of the essential mitochondrial metabolite L-carnitine that is catabolized by plasma esterases into carnitine. Acetyl-L-carnitine facilitates the uptake of acetyl-CoA into mitochondria during fatty acid oxidation, enhances acetylcholine production, and stimulates protein and membrane phospholipid synthesis. In vivo, acetyl-L-carnitine (100 mg/kg) increases mGlu2/3 receptor protein levels and mechanical pain thresholds in a mouse model of chronic inflammatory pain induced by complete Freund's adjuvant.