Five epileptic women needing to commence phenytoin therapy during pregnancy received a single intravenous and a single oral dose of phenytoin several days apart before starting regular intake of the drug. Plasma phenytoin concentration - time data were analysed by three different pharmacokinetic techniques. However assessed, the mean oral bioavailability of the drug proved to be about 90% of the intravenous bioavailability. This finding makes it unlikely that impaired bioavailability accounts for the increase in oral phenytoin dosage necessary in pregnancy to maintain plasma phenytoin concentrations at pre-pregnancy values. Phenytoin clearance in the pregnant subjects was approximately double the published values for phenytoin clearance in non-pregnant persons. This suggests that increased (metabolic) clearance accounts for the increased phenytoin dosage requirement of pregnancy.