1. After oral administration of [14C]dihydropyridine diester, the plasma concn. of radioactivity was similar in rats and dogs, reaching a maximum at 0-5 to 1 h and decreasing with a half life of about 3-5 h. The plasma concn. of unmetabolized drug in dogs was 10 times higher than in rats. Radioactivity in rat tissue was high in liver, kidney and lung after both oral and intravenous administration. 2. In both species, 66-72% of radioactivity was excreted in faeces and 23-29% in urine in 48 h, regardless of the route of administration. Biliary excretion in rats after oral dosage amounted to 65%. 3. Eight metabolites were identified from urine of dogs and rats. They were derived from one or several of the following pathways: I, debenzylation of the N-benzyl-N-methylaminoethyl side chain; II, reduction of the 3-nitro group on the phenyl substituent; III, oxidation of the 1,4-dihydropyridine ring to the corresponding pyridine; IV, oxidative removal of the N-benzyl-N-methylamino group yielding a carboxylic acid; V, hydrolysis of the N-benzyl-N-methylamino-ethyl ester to the corresponding carboxylic acid; VI, hydroxylation of the 2-methyl group of the 1,4-dihydropyridine ring to hydroxymethyl.