The metabolic fates of N^G,N^G- and N^G,N^G-dimethylarginines in rats were investigated isotopically and novel metabolites, α-keto-δ-(N,N-dimethylguanidino)-and α-keto-δ-(N,N′-dimethylguanidino) valeric acids and γ-(N,N-dimethylguanidino)-and γ-(N,N′-dimethylguanidino) butyric acids were identified. In the case of the rats injected with N^G,N^G-dimethyl-l-[1,2,3,4,5-¹⁴C]arginine, about 13% of the radioactivity was recovered in the first 12-h urine and was distributed in the following metabolites (relative ratios): unchanged N^G,N^G-dimethyl-l-arginine (35.2%), γ-(N,N-dimethylguanidino)butyric acid (18.4%), α-keto-δ-(N,N-dimethylguanidino)valeric acid (16.4%), and N-acetyl-N^G,N^G-dimethyl-l-arginine (8.5%). The radioactivity retained in the tissues was found mainly in citrulline and was further distributed in ornithine, arginine, and glutamic acid and even in protein-bound arginine. In the case of N^G,N′^G-dimethyl-l-[1,2,3,4,5-¹⁴C]arginine-injected rats, about 75% of the radioactivity was excreted in the first 12-h urine and was recovered in the following metabolites (relative ratios): N^α-acetyl-N^G,N′^G-dimethyl-l-arginine (48.4%),unchanged N^G,N′^G-dimethyl-l-arginine (23.7%), α-keto-δ-(N,N′-dimethylguanidino) valeric acid (20.2%), and γ-(N,N′-dimethylguanidino)butyric acid (9.6%). In the tissues, most of the radioactivity was associated with unchanged N^G,N′^G-dimethyl-l-arginine. These findings show that both dimethylarginines are metabolized by a pathway forming the corresponding α-ketoacid analogs and the oxidatively decarboxylated products of the α-ketoacids in addition to the N^α-acetyl conjugates identified previously (K. Sasaoka, T. Ogawa, and M. Kimoto (1982)Arch. Biochem. Biophys.219, 454–458), and N^G,N^G-dimethyl-l-arginine is catabolized by an additional pathway leading to the formation of citrulline and its metabolically related amino acids. By considering their catabolism, an attempt to use urinary dimethylarginines as an index of in vivo breakdown of tissue proteins is invalid at least in rats.