The distribution of phenobarbital (PB) in whole blood and its fractions in 8 pregnant women, in 10 mothers and infant pairs at delivery and in 7 controls was studied by equilibrium dialysis. The lowest protein binding was in cord plasma. A correlation was found between the maternal and cord plasma PB binding. The adult plasma PB binding ratio was correlated with the molar albumin concentration, but no such relation was found in cord plasma, suggesting inhibition of binding by other factors. As a counterpart to the lower protein binding in maternal and cord plasma, the fraction of PB distributed to erythrocytes in those groups was significantly higher. The apparent concentration of PB in maternal erythrocytes was increased. The distribution of drugs to the various fractions of whole blood is an important consideration perinatally, since the binding characteristics of plasma and blood cells in mother and fetus will be crucial determinants for the placental transfer of drugs.