Pregnant women were treated with infusions of acetylsalicylic acid (ASA). The plasma concentrations of total salicylate or those of ASA and salicylic acid (SA) were determined. The ASA steady state was reached after 2 h of infusion whereas 3 days seemed necessary to reach the steady state of SA. In a further study during the late phase of cervical dilatation (12 min - 2 h before birth) healthy pregnant women were given 1 g ASA as a single i.v. bolus injection. The resulting plasma levels of ASA and SA in the mothers during delivery, the plasma concentrations in umbilical cord blood and in blood obtained from the newborn several hours after birth were measured. With increasing time the SA concentrations in the umbilical cord blood plasma approached those of the mothers. On the other hand, the umbilical cord blood concentrations of ASA did not approach the maternal concentrations, probably because of the ASA esterase activity at the placental barrier. The plasma concentrations in the newborn indicated that the newborn is able to degrade ASA and to excrete SA. However, the velocities are lower than the respective velocities in adults.