1. The metabolism of diethyl 4-[(4-bromo-2-cyanophenyl)carbamoyl]benzylphosphonate (NO-1886), an antilipidaemic agent, was evaluated in the rat. 2. 14C-NO-1886 was dosed orally to rats (3 mg/kg) and within 24 h after dosing, 27.9 +/- 2.1 and 63.5 +/- 4.2% of the administered radioactivity was recovered from urine and faeces respectively. 3. The metabolite M-2 was isolated from the urine and faeces, and two other metabolites, M-3 and M-5, were isolated from the urine. Two of them were identified as ethyl 4-[(4-bromo-2-cyanophenyl)carbamoyl]benzylphosphonate (M-2) and 4-[(diethoxy-phosphoryl)methyl)]benzoic acid (M-3), and the other one was considered to be 2-amino-5-bromo-3-cyanobenzene sulphate (M-5) by ms and nmr spectrometry. 4. The major metabolic pathway of NO-1886 was found to be mono-hydrolysis of the diethyl phosphonate. It was also considered that M-5 may have been formed in vivo via 2-amino-5-bromo-benzonitrile (M-1) and 2-amino-5-bromo-3-hydroxybenzonitrile (M-4).