Phenytoin, phenobarbitone and primidone in small samples of blood can be identified and measured simultaneously. The acidified blood is extracted with 3 ml of diethyl ether and 2 ml of the extract are evaporated to dryness. The residue is dissolved in 80 µl of diethyl ether containing methyltesto-sterone (15 mg per 100 ml) as the internal standard. An aliquot of this solution is injected on to an SE-30 column where the drugs are separated and detected by a flame-ionisation detector. Drugs commonly associated in treatment with these three and cholesterol, which is co-extracted, do not interfere.