Eighteen pregnant women were treated with aspirin, 37.5 mg once daily by mouth. Treatment was started two weeks before the expected date of delivery, and continued until delivery. Seventeen untreated women were studied concurrently. Platelet thromboxane (TX) production was determined by radioimmunoassay of TXB2 in serum from blood incubated for one hour with thrombin at 37 degrees C. Maternal blood was studied before treatment and at delivery. Fetal blood, from the cord, was studied at delivery. Prostacyclin (PGI2) production by rings of umbilical artery incubated in Hanks' solution at 37 degrees C for one hour was determined by radio-immunoassay of its hydrolysis product, 6-oxo-prostaglandin (PG) F1 alpha. Maternal and fetal blood from untreated women produced similar amounts of TXB2. Aspirin, in the dose regimen used, significantly inhibited TXB2 production in maternal but not in fetal blood, and did not impair PGI2 synthesis by umbilical artery rings. This differential effect on the cyclo-oxygenase of maternal platelets is probably due to the unusual kinetic properties of aspirin, and may prove therapeutically useful.