The present study was performed to determine the metabolic fate of the residues of diazepam and its metabolites in an in vivo model and using two different animal species, guinea pigs and rats, successively. Guinea pigs were orally dosed with diazepam at 100 mg/kg bw and 10 mg/kg bw. After 2 h the animals were sacrificed and their livers were removed. Rats were fed these livers and sacrificed 8 h later after having ingested the portion given. Blood, kidney, and liver of rats were collected to quantify drug residues. Oxazepam and demethyldiazepam were identified by HPLC-UV in liver and kidney rat extracts and also in some blood samples. Temazepam was only found in liver from rats that had eaten guinea pig liver dosed at the highest level. The identity of demethyldiazepam was confirmed in rat liver extracts by GC-MS. This study demonstrates the bioavailability of diazepam metabolites in a second animal (rat simulating a consumer) following a diazepam administration to a first animal (guinea pigs simulating a target animal). Residue concentrations were reduced from 9.7 and 243 micrograms in consumed guinea pig liver to 3.75 and 455 ng/g in rat liver for parent drug and oxazepam, the lowest and highest residues found, respectively.