Little information is available regarding appropriate plasma levels of anticonvulsant drugs in neonates. We determined the plasma levels of phenobarbital and diphenylhydantoin following initial administration and during maintenance therapy in 59 neonates with seizures. Following intravenous administration of 15 to 20 mg/kg, levels of 20.7 +/- 4.4 microgram/ml were achieved for phenobarbital and levels of 14.5 +/- 3 microgram/ml for DPH. Maintenance doses of 5 mg/kg of phenobarbital resulted in initial drug accumulation followed by more rapid elimination of the drug with increasing duration of exposure. Therapeutic plasma levels of DPH could not be achieved by oral administration in the neonate.