The antiepileptic drug valproic acid was studied in an open clinical trial as adjunct medication for 23 patients with uncontrolled seizures of a generalized or partial type. Two-thirds of the patients experienced reduction in seizure frequency ranging from 25 to 100%. Extensive testing revealed no evidence of serious systemic toxicity due to the drug. Minor side effects (e.g., nausea, vomiting, or sedation) were usually transient. Sodium valproate syrup and valproic acid in capsules gave equivalent mean low (23.3 microgram/ml) and maximum (42.5 microgram/ml) serum concentrations. The drug had a relatively short half-life of 8.7 hours, necessitating administration in divided daily doses. During initiation of valproate therapy there was evidence of a decline in total serum phenytoin concentration (16.5 to 10.2 microgram/ml; p less than 0.001) while the percentage of free phenytoin increased (10.9 to 20%). The quantity of unbound phenytoin was relatively stable throughout. This observation was interpreted as a drug interaction: valproic acid competed with phenytoin for access to plasma protein binding sites.