Possible factors affecting the seizure frequency during pregnancy and puerperium were prospectively studied on 125 cases, paying particular attention to the drug compliance and the antiepileptic drug disposition. Of the subjects, 27% were under the poor compliance category. As the reasons for the poor compliance, anxiety about the side effects of antiepileptic drugs including teratogenecity and the harmful effects on their newborns by breast feeding were found in about half of them. In the cases with regular drug taking, 80% showed no change in the seizure frequency whereas only 16% exhibited an increase and 4% a decrease. The apparent serum clearance of phenytoin, primidone, phenobarbitone derived from primidone and valproic acid increased during pregnancy and puerperium. In some cases, the lowering of the serum level of drugs was associated with the aggravation of seizures.