In the treatment of epilepsy often several substances with anticonvulsive effect are combined. Possible drug interactions in these cases can change the desired effect of treatment. Simultaneous administration of clonazepam or dipropylacetate (the latter in a short term combination) with diphenylhydantoin can cause a significant increase of diphenylhydantoin serum concentrations and intoxications. The combination of carbamazepin with diphenylhydantoin can cause a decrease of diphenylhydantoin serum concentrations. The simultaneous administration of diphenylhydantoin and phenobarbital can produce a significant increase of phenobarbital levels in the statistical average and in the case of a combination of primidon and diphenylhydantoin an intoxication by the primidon metabolite phenobarbital. These possible interactions which are not obvious at the beginning of therapy are supplemented by other factors as intercurrent diseases or erratic drug intake. With routine measurements of serum concentrations of anticonvulsive drugs some of these interfering factors can be eliminated by realizing them in time. Treatment becomes more effective and side effects are reduced. The development of a new check list for the treatment of epileptic patients should also improve the control and give better informations about the course of the disease.