Cytochrome P450 expression in liver is influenced by several factors, including species, sex and strain. We compared metabolism formation of clozapine in different species (rat, mouse, guinea-pig, dog, monkey and man) so as to choose between species to further validate interaction studies. Liver microsomes of male and female Sprague-Dawley rats, hairless rats, OF1 mice, Balb C mice and Dunkin-Hartley albino guinea-pigs, male beagle dogs, male cynomolgus monkeys and man were used to investigate in vitro metabolism of clozapine. This process was dependent on the presence of NADPH and on the presence of microsome protein. In addition, we observed the formation of desmethyl- and N-oxide metabolites, with the rate of formation of each of these compounds varying with species, sex and strain of microsomes incubated. The desmethyl- and N-oxide metabolites formed were statistically greater in male than in female rats, mice in the two strains studied, as well as for the guinea-pigs. Levels of desmethyl clozapine formed were high for the rats and no significant difference in clozapine biotransformation was observed between Sprague-Dawley and hairless rats. For man, the formation of metabolites of clozapine was comparable with guinea-pig, dog and monkey. In addition, we screened the effect of 52 molecules, representative of 11 different therapeutic classes, on the metabolism of clozapine by rat liver microsomes. We found that most of the calcium channel blockers (diltiazem, felodipine, isradipine, lacidipine, nicardipine and nitrendipine), antifungals (ketoconazole, miconazole) and two anticancer drugs (paclitaxel, teniposide) caused more than 50% inhibition of clozapine metabolism in vitro. The extent of inhibition was increased in a concentration-dependant manner. Complementary clinical and pharmacokinetic studies should be performed to confirm these results.