Phenobarbital pretreatment of pregnant mice (ASH/TO strain) gave rise to approximately equal concentrations of phenobarbital in both maternal and fetal liver. This pretreatment resulted in increased UDP-glucuronyltransferase (GT) activity towards bilirubin in maternal and neonatal liver and in fetal liver on days 19 and 20 but in livers from earlier (15–18 day) fetuses GT either was not significantly increased or remained undetectable. Fetal liver is thus not competent to respond to phenobarbital by increasing its GT activity, until just before birth. This pattern persisted through changes in assay conditions and is contrasted with that occurring in embryos free from maternal influence. GT from adult and neonatal liver is activated by 0.2% digitonin; in fetal liver this response also does not appear until day 19.