Phenytoin serum concentrations were evaluated in 88 epileptic women at different stages of pregnancy and 40 women during postnatal periods. In addition, concentrations were determined from the umbilical cords of 27 neonates. On average, the dose of phenytoin was increased by 130 +/- 54 mg in 67% of the patients in order to control seizures. In 76% of the women during pregnancy and 95% in the postnatal periods, dose adjustment was achieved for the control of the seizures. Therapeutic clinical concentrations ( < 9.9 micrograms/ml) were found in 64% of the patients, with an average of 7.2 +/- 1.8 micrograms/ml during pregnancy and 6.2 +/- 2 micrograms/ml in 90% of the women during the postnatal period. The average phenytoin concentration reached with doses of 100, 200, 300, 400 and 500 mg were 3.3, 5.7, 8.4, 10.8, and 14.1 microliters/ml, respectively, without statistically significant differences among the pharmacokinetic parameters measured during pregnancy, between pregnancy and the postnatal period. The proportion between fetal and maternal phenytoin concentration was 0.37 +/- 0.28. Hydantoin fetal syndrome was seen in 8% of the neonates, without a statistically significant difference among patients with or without seizures. No relation was found between the concentration of phenytoin during pregnancy and the hydantoin fetal syndrome. The study shows that low concentrations of phenytoin can control seizures during pregnancy and the postnatal period and the need to relate serum phenytoin concentrations with the clinical state of pregnant women who suffer seizures.