Abstract Following the intraperitoneal injection of l-homocitrulline into rats a previously undescribed metabolite of this amino acid was detected in the urine. A compound with identical properties could also be prepared with l-amino acid oxidase. Mass spectrometric studies indicated that both of these derivatives had a molecular weight of 170.0690 and an empirical formula of C 7H 10O 3N 2. Upon exposure to acidic conditions the compound was converted a second form which had a molecular weight of 152.0585 and an empirical formula of C 7H 8O 2N 2. Nuclear magnetic resonance studies confirmed the cyclic structure of the unknown.