The lower limits of the "therapeutic range" for serum levels of sodium valproate (VPA) were evaluated in epileptic children showing a benign clinical course. Twenty-five outpatients, aged 5 to 16 years, whose seizures were well controlled over three years with VPA alone, were studied. Venous blood was taken 1.1 to 5 hrs after the morning dose. Serum VPA concentrations at steady-state after receiving the maintenance doses to control seizures were determined by enzyme immunoassay. The patients were divided into three groups according to the seizure type and the age at onset; A and B: patients with tonic and/or clonic seizures, aged below 3 yrs (n = 11) and 3 to 11 yrs (n = 6), respectively, C: those with absence seizures, aged 4 to 11 yrs (n = 8). The serum concentrations in A (47.8 +/- 21.6 micrograms/ml, mean +/- SD) were significantly (p less than 0.02) lower than those in groups B and C (85.2 +/- 14.0 and 73.0 +/- 17.4 micrograms/ml, respectively). VPA concentrations below 50 micrograms/ml were seen in 6 patients (55%) in group A. It was concluded that many epileptic children, whose ages at onset were below 3 yrs, with tonic and/or clonic seizures could be controlled even with low initial serum concentrations below the "therapeutic range".