A prospective randomized study was performed to investigate the effect of a short-term ritodrine infusion on the concentrations of maternal and fetal prostaglandins and their precursor fatty acids. Mothers gave birth by an elective cesarean section at term, and either ritodrine (study group) or physiological saline (controls) was infused 2 h prior to the operation. Ritodrine decreased the levels of thromboxane A2 significantly in the mother, while the concentration of prostacyclin remained unchanged. In the umbilical arterial plasma concentrations of prostacyclin or thromboxane A2 there were no differences between ritodrine-treated and control subjects. Ritodrine had no effect on the prostaglandin precursor fatty acids and other fatty acids of plasma phospholipids of the mother or her fetus. It is concluded that ritodrine may suppress the maternal prostaglandin production, but has no effect on the levels of vasoactive prostaglandins or their precursor fatty acids in the fetus.