Phenobarbital and phenytoin binding profiles were determined in 27 neonates. Binding of both drugs decreased compared with that in older subjects. In vitro binding of both agents correlated significantly with total protein and albumin concentrations. In vivo binding at 0.5 hours correlated significantly with birthweight and gestational age. Phenobarbital, but not phenytoin, binding decreased when three other therapeutic agents were concomitantly administered. Bilirubin concentrations, free fatty-acid concentrations, and pH values encountered in this population did not significantly influence binding. An in vitro binding profile accurately predicted in vivo free fractions (percent drug unbound) and plasma concentrations of both drugs.