The relationship of the initial phenobarbital dose to weight, gestational age, blood level, and seizure control was studied in 39 neonates. The blood proportional to the dosage per kilogram, and was not related to weight or gestational age. Seizures remitted only at blood phenobarbital concentrations above 16.9 micrograms per milliliter. Therapeutic levels can be achieved by the intravenous or intramuscular administration of 16 to 23 mg per kilogram of phenobarbital.