The excretion of three oestrogen fractions and progesterone metabolites in 64 female epileptic patients was determined during the menstrual cycle, and in 50 women of this sample serum phenytoin and phenobarbitone levels were measured. A significant decrease of both hormones in epileptic patients was found as compared to a control group. The variations in serum phenytoin levels were greater in females with so-called catamenial epilepsy with a marked fall of drug levels between days 27 and 28 corresponding with an increase of seizure frequency. The effect of progesterone deficit on seizure susceptibility before menstrual bleeding is discussed, and the need of serum anticonvulsant level determination during the premenstrual phase in epileptic women is suggested.