Plasma anticonvulsant levels were followed during pregnancy in 11 epileptic women taking phyenytoin and/or phenobarbital or a drug metabolized in the body to phenobarbital. From the relationship between plasma level and drug dose, phenytoin requirement increased in all 10 women taking this drug during pregnancy. The requirement fell again in the puerperium. Plasma phenobarbital levels decreased during pregnancy in all 5 women taking a constant daily dose of phenobarbital or a congener. These findings should be considered if epileptics are to be protected against seizures during pregnancy and against anticonvulsant overdosage during the puerperium.