Phenobarbital disposition in the neonate.

The disposition of phenobarbital in premature infants was assessed by repeated measurement of the serum concentration of unmetabolized drug and the urinary excretion of both phenobarbital and its metabolite, p-hydroxyphenobarbital. Serum elimination rates did not adequately reflect the elimination of total body phenobarbital. A prolonged distributive phase is postulated. The elimination rate of p-hydroxyphenobarbital was constant, suggesting capacity limited kinetics. Slow elimination in serum concentrations which remained in the therapeutic range for several days suggests that short courses of therapy may have prolonged effect in the newborn premature infant.