To study the effect of nicotine on fetal prostacyclin and thromboxane A2, specimens from the umbilical arteries of infants born to healthy nonsmoking mothers were superfused in the absence or presence of nicotine (50 to 10,000 micrograms/mL), and the releases of 6-keto-prostaglandin F1alpha (a break-down product of prostacyclin) and thromboxane B2 (a metabolite of thromboxane A2) were measured. The baseline production of 6-keto-prostaglandin F1alpha (63.9 +/- 8.8 ng/minute per gram of dry weight tissue, mean +/- SE, N = 10) or that of thromboxane B2 (1.3 +/- 0.2 ng/minute per gram, N = 10) were unaffected by nicotine. To study the effect of nicotine on thromboxane A2 synthesis by the fetal platelets, thrombin-induced platelet aggregation and consequent thromboxane A2 synthesis were allowed to occur in the whole cord blood in the absence or presence of nicotine (10 to 500 micrograms/mL). Nicotine inhibited concentration dependently platelet thromboxane A2 synthesis from the baseline level (107.3 +/- 7.1 ng/mL) by 15 to 93%. This inhibition was also seen in thromboxane A2 synthesis starting from exogenous arachidonic acid, suggesting that nicotine inhibits either cyclooxygenase and/or thromboxane A2 synthetase in the fetal platelets. Thus, nicotine is hardly responsible for maternal smoking-induced changes in fetal prostacyclin formation.