NG,NG-dimethyl-L-arginine-d6 (ADMA-d6) (hydrochloride) is intended for use as an internal standard for the quantification of NG,NG-dimethyl-L-arginine by GC- or LC-MS. ADMA is an endogenous inhibitor of nitric oxide synthase (NOS). It is formed from arginine by protein arginine methyltransferases (PRMTs) and degraded by dimethylarginine dimethylaminohydrolases (DDAHs) and alanine-glyoxylate aminotransferase 2 (AGXT2). ADMA levels are increased concomitant with an increase in blood pressure in Dahl salt-sensitive rats fed a high-salt diet. ADMA levels are increased in the plasma in a variety of endothelial dysfunction-related diseases, including hypertension, congestive heart failure, and end-stage renal disease.