The effect of carbamezepine and phenytoin on the serum level of clonazepam and its major metabolite 7- aminoclonazepam was studied in epileptic patients chronically treated with combination of those drugs. Carbamazepine produced consistent but insignificant lowering of the clonazepam serum level which averaged 20% wiithout noticeable effect on the 7- aminoclonazepam. Phenytoin however produced significant reductions in clonazepam with slight lowering of 7- aminoclonazepam.The present finding indicates that phenytoin and to less extent carbamezapine lower the clonazepam serum level. the finding also suggests that the effect may be due to enhanced metabolism of clonazepam via transformation into 7- aminoclonazepam