Loxapine is a dibenzoxazepine neuroleptic that is metabolized by the liver in humans. In the present study, we investigated first in vitro loxapine metabolism in liver microsomes from various species including rats, mice, guinea pigs, dogs, rabbits, monkeys and humans. This enables us to choose between species to further validate drug-drug interaction studies. We observed the formation of desmethyl- and hydroxy- metabolites of loxapine after incubation of the different species liver microsomes. Hydroxylation pathway was major in all species. Wide interspecies variability of loxapine metabolism was observed. Loxapine metabolism was similar in human, guinea pig and dog microsomes. We screened in vitro effects of 67 molecules, representative of 8 therapeutic classes, on loxapine metabolism. Loxapine (100 microM) was incubated with guinea pig liver microsomes (1 mg/ml) 30 min at 37 degrees C with and without the presence of interacting drug. We found that most of psychotropics (alimemazine, cyamemazine and levomepromazine), antifungal (ketoconazole), anticancer drugs (daunorubicin, pirarubicin) and analgesic (nefopam) inhibited more than 50% of hydroxyloxapine formation in vitro. Complementary clinical and pharmacokinetic studies should be performed to confirm these results.