1. Transplacental transfer of the anti-epileptic agent phenytoin (PHT), its phase I metabolite, p-hydroxy-phenytoin (p-OH-PHT), and its phase II conjugate p-OH-PHT-glucuronide, was studied in term placental lobules perfused single pass in both maternal and fetal circuits. 2. Ratios of clearance of PHT, p-OH-PHT and p-OH-PHT-glucuronide to clearance of antipyrine were 1.08 +/- 0.03, 0.52 +/- 0.02 and 0.12 +/- 0.01 (mean and s.e.m.), respectively. 3. Transfer was positively correlated with lipophilicity as measured by the apparent partition coefficient determined between octanol and pH 7.4 buffer.