The anticonvulsant, sodium valproate (Epilim), is currently undergoing clinical trials in Australia. Since the estimation of plasma levels of anticonvulsants now plays an important role in the treatment of epilepsy, we have developed a rapid assay for plasma levels of sodium valproate. The 1-step extraction procedure involves the addition of the internal standard, cyclopentane carboxylic acid, to the plasma sample and subsequent extraction into chloroform. Sodium valproate is a branched-chain fatty acid. We determine free fatty acids by gas liquid chromatography using the stationary phase, 10% SP 216 PS, one of the most polar presently available. This property enables fatty acids, including sodium valproate, to be estimated without prior derivatization. After extraction of 200 μl plasma, the limit of detection of sodium valproate by this method is 20 μmol/l. A group of patients treated with sodium valproate has been studied.