123 plasma concentration measurements of phenobarbital were obtained from 82 children (2 months - 6 1/2 years old) at steady-state conditions. The plasma level/dosage ratio has been found to have a highly significant correlation with the age of the patient both for dosage in mg/kg and in mg/m2. The ratio increases with the increase in the age of the patient at a rate which is greater for dosages expressed on the basis of body weight. Moreover, at least for body weight related dosages, this increase is relatively high in the first year of life, becoming less marked after. Practical indications are given about the required dosage of phenobarbital in different groups of ages from 2 months to 6 1/2 years. It is recommended however to regularly measure the plasma level of the drug in infants and children treated for long periods of time.