Diazepam (DZ) was used as a substrate in drug metabolism studies to characterise the differences in metabolite profiles in hepatocytes isolated from four species: Wistar rat, cynomolgus monkey, beagle dog and man. Hepatocytes were incubated with DZ (20 microM) for 180 min at 3 hr post isolation in culture, and the disappearance of parent compound and appearance of its metabolites determined. DZ disappearance was found to be monoexponential in rat, monkey and human cells, but that DZ disappearance in dog hepatocytes was best described by a two compartment process. There were considerable differences in both the rates of formation and the profiles of metabolites produced from DZ in each species. Drug metabolism of DZ was determined in five human hepatocyte preparations. The rates of formation of both the major metabolites, temezepam (TEM) and nordiazepam (NOR) were highly variable between samples, and oxazepam (OX) was detected in only three of the preparations. There was no evidence of further metabolism of these metabolites, and the profiles were comparable with in vivo findings. In a single case, human hepatocytes were cultured for five days, and DZ was used as substrate to characterise the changes in drug metabolising activities. There was a rapid loss in the production of OX in the initial 24 hr, and a complete loss of 3-hydroxylation activities in the succeeding 120 hr. N-demethylation activities, however, were well maintained, and the appearance of NOR declined to 47% of initial rate. The hepatocytes of all species were found to produce NOR and TEM as metabolites; NOR representing the principal metabolite in the dog, monkey and human cells. In the dog, TEM was found only as a minor metabolite. OX was a significant metabolite in the monkey and a minor metabolite in the dog and human hepatocytes, but was not detected in rat cultures. The principal metabolite in rat cells was 4'-hydroxy diazepam, which was rapidly further metabolised to its glucuronide. The drug metabolising activities of the hepatocyte cultures towards DZ were comparable with the drug metabolism of DZ found in vivo in each species. These findings substantiate hepatocytes as an in vitro model of hepatic metabolism.