The protein binding of diazepam, desmethyldiazepam, furosemide, indomethacin, warfarin, and phenobarbital in maternal and fetal cord serum at the time of birth, and in serum of neonates between 1 and 11 days of age was studied. The protein binding of diazepam and desmethyldiazepam was higher in the fetus than in the mother, thus explaining the fetal cumulation of these drugs in vivo. After birth, both drugs were partially displaced from neonatal binding sites. The decreased protein binding capacity in the mother and the neonate related to increased free fatty-acid levels. The pattern of protein binding of warfarin in the groups investigated was a mirror image of those of diazepam and its metabolite. The protein binding of indomethacin progressively decreased in the neonate during the first two postnatal weeks, while that of furosemide remained at lowered levels throughout this time interval. The protein binding of phenobarbital was similar in the groups investigated. Our results suggest that drugs such as diazepam, which can be displaced from binding sites by free fatty acids, may cumulate in the fetus and may exhibit much decreased protein binding and possibly unexpectedly strong effects in the neonate after birth.