We have studied effects of in vivo lead acetate (Pd) exposure on barrier maturation. Rabbit fetuses, newborn and adults received 20 mg/kg Pb for 5 days before sacrifice (or before delivery of pups). No age-dependent or Pb-associated changes occurred in CSF or plasma Na levels. Na-K ATPase activity (micromoles Pi/mg protein/hour) rose significantly with age in brain and activity was similar at all ages studied in choroid plexus. Pb decreased ATPase activity only in brain at 30 days. The results show striking regional and temporal diversity in barrier maturation. Both the electrolyte gradients and transport enzyme studies suggest that the choroid portion of the barrier system is functional by the late fetal period, anticipating by several weeks maturation of the corresponding neural systems. This situation influences the action of Pb which affects function of the blood-brain barrier only after its complete maturation.