The transplacental and elimination pharmacokinetics of phenobarbital administered antenatally was analyzed in both mothers and infants as part of a study evaluating the efficacy of antenatally administered phenobarbital in the prevention of neonatal intracerebral hemorrhage. Twenty-five pregnant women in labor less than 35 weeks' gestation received 500 mg phenobarbital administered intravenously. The maternal serum phenobarbital level at delivery was 8.76 +/- 1.99 micrograms/ml and cord serum phenobarbital level was 9.0 +/- 1.75 micrograms/ml (all values are mean +/- SD). There was no correlation between the time from phenobarbital administration to delivery (5.6 +/- 4.6 h) and the cord:maternal serum phenobarbital ratio (1.05 +/- 0.21) (r = -0.03; p greater than 0.05). The mean apparent half-life of phenobarbital estimated in 11 infants was 175.5 +/- 45.6 h. The present study documents the ability to predict serum levels in the fetus and neonate from the serum concentrations achieved in the mother.