A 12-week study of clinical response, EEG changes and serum antiepileptic drug (AED) levels using sodium valproate (VAL) was undertaken. The study showed that VAL is a powerful adjunct in the treatment of intractable epilepsy. It was most effective in patients with generalized seizures, but no seizure type was totally resistant. No serious adverse effects were encountered; nausea was easily overcome by readjusting the drug dosage. In most cases the only EEG change was decrease of epileptiform activity, and this correlated well with decreased frequency of clinical seizures. These two features in turn were most often seen with a serum VAL level of 40 μg per milliliter or greater. Intoxication with VAL was accompanied by marked slowing of the background rhythms, but no increase in beta activity. Other modifications of the EEG were probably due to changes in the plasma levels of other drugs. Interactions between VAL and conventional antiepileptic drugs occur, so that serum concentrations of all drugs must be monitored in patients receiving VAL.