The concentrations of both total and free phenytoin in the plasma of epileptic women tend to decrease during pregnancy, suggestive of a pregnancy-associated increase in the metabolic clearance of the drug. On the other hand, the metabolic clearance of free (unbound) phenytoin decreases during pregnancy in rats. One possible reason for this species difference is the routine dietary supplementation of folic acid in human pregnancy and the apparent ability of folic acid to lower phenytoin plasma concentrations even in nonpregnant humans. The purpose of this investigation was to determine the effect of treatment with folic acid on the pharmacokinetics of phenytoin in pregnant and female nonpregnant rats. In one experiment, the treated animals received folic acid in the drinking water, approximately 100-150 micrograms/kg/d, for 19 d. There was no apparent difference between the treated and untreated rats in the pharmacokinetics of a 10-mg/kg iv dose of phenytoin (which was administered to the pregnant rats on the 20th day of gestation), regardless of pregnancy status, In another experiment, pregnant and female nonpregnant rats received either folic acid, 400 micrograms/kg/d, or an equal volume of the solvent only, by gastric intubation for 19 d. The next day (which was the 20th day of gestation for the pregnant rats), the animals received an intravenous injection of phenytoin, 30 mg/kg. Again, pretreatment with folic acid had no apparent effect on the pharmacokinetics of phenytoin in both pregnant and nonpregnant rats. However, the results of this investigation confirm previous observations of dose-dependent phenytoin pharmacokinetics in rats and of decreased clearance of free phenytoin in late pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)