Sodium valproate (VPA) was first marketed in the United States in 1978. In this pilot study of pharmacokinetics and toxicity, VPA was added to the treatment regimens of 20 patients (10 adults and 10 children) with intractable seizures. The drug was absorbed and excreted rapidly; the mean half-life was 9.6 hours. Drowsiness and gastrointestinal symptoms were the most common side effects, but they were usually minor and transient. An increase in some plasma phenobarbital levels and a decrease in some plasma phenytoin levels were attributed to drug interaction. Control of absence attacks was assessed by 12-hour telemetered electroencephalograms. Sodium valproate was most efficacious in generalized seizure disorders, particularly absence seizures.