Blood levels of phenobarbital were determined after a single oral or i.m. dose in children in the hospital after febrile convulsions. At a dose of 15 mg/kg, both the oral and i.m. routes gave therapeutic blood levels within 90 min. Absorption from the i.m. route before 90 min was inconsistent and would be unlikely to arrest an established convulsion within a critical time period. For use as a drug to prevent convulsions, oral phenobarbital at 15 mg/kg requires further study.