The placental transfer of phenobarbital was investigated in 35 mother-infant pairs at birth. The drug was administered prenatally to the mothers for maternal epilepsy (group 1, n = 5), gestational hypertension and preeclampsia (group 2, n = 20) and prophylaxis of intraventricular hemorrhage in premature deliveries (group 3, n = 10). The phenobarbital levels in arterial cord blood were 100 +/- 2.8% in group 1, 89 +/- 21% in group 2 and 77 +/- 16% in group 3 with respect to the levels observed in the mothers. The most important factor influencing the transplacental passage was the duration of maternal treatment in the infant of group 1 (r = 0.80, p < 0.01), the gestational age in the infants of group 2 (r = 0.74, p < 0.01) and the arterial cord pH in the infants of group 3 (r = 0.89, p < 0.001).