1. After oral administration of aristolochic acid I (AAI) and aristolochic acid II (AAII) to rats, the following metabolites were isolated from the urine and their structures elucidated: aristolactam I, aristolactam Ia, aristolochic acid Ia, aristolic acid I and 3,4-methylenedioxy-8-hydroxy-1-phenanthrenecarboxylic acid (metabolites of AAI); or aristolactam Ia, aristolactam II and 3,4-methylenedioxy-1-phenanthrenecarboxylic acid (metabolites of AAII). A further metabolite of AAII having a lactam structure has not yet been isolated in pure form. 2. The metabolic pathways have been elucidated by administration of various metabolites. 3. The principal metabolite of AAI in rats was aristolactam Ia; 46% of the dose was excreted in the urine in form of this metabolite and 37% in the faeces. The other substances were minor metabolites. Those metabolites of AAII whose structures have been elucidated were minor metabolites; the largest proportion consisted of aristolactam II, which accounted for 4.6% in the urine and 8.9% in the faeces. 4. The mouse was the only animal which had the same metabolite patterns of AAI and AAII as those found in the rat. Not all the metabolites listed above were found in urine from guinea pigs, rabbits, dogs and man.