The mentally retarded often need concomitant antiepileptic and neuroleptic drug treatment. High doses of neuroleptic drugs may provoke seizures. Antiepileptic drugs may aggravate behavioural problems. The mutual influence of neuroleptic and antiepileptic drug treatment and the effect of seizure control were studied in 20 mentally retarded patients between 1980 and 1989. The treatment was tailored individually, aiming at the lowest effective dose. Carbamazepine was preferred to phenobarbital and phenytoin. The mean defined daily dose (DDD) of neuroleptics and antiepileptics was reduced by 64% and 5%, respectively. Changing the mean DDD of neuroleptics neither correlated significantly with seizure activity nor with the change of the mean DDD of the antiepileptics. Evidently, seizure control may be improved by small neuroleptic doses in some patients. In one patient, however, a non-convulsive status epilepticus was associated with the introduction of neuroleptics. The assumption that carbamazepine has a beneficial effect on behavioural problems was not supported. Apparently, changing the regime of antiepileptics contributed to less neuroleptic requirements, possibly through reduced side-effects and/or improved seizure control. When combining neuroleptics and antiepileptics, interactions should always be considered. The epileptogenic effect of small to standard doses of neuroleptic drugs should, however, not be overemphasized.