Antipyrine and 4-aminoantipyrine have been used for approximately 20 years to measure uterine and umbilical blood flow. Fetal infusion of 4-aminoantipyrine has been shown to decrease myometrial activity and to significantly lower prostaglandin F2 alpha metabolite levels. Since prostaglandins are thought to be important in regulating uterine and umbilical blood flow, their decrease could cause significant changes in blood flow. The purpose of the present study was to evaluate the effects of antipyrine on uterine and umbilical blood flow as measured with electromagnetic flow probes and to determine whether antipyrine causes significant changes in levels of prostaglandin E2, prostaglandin F2 alpha metabolite, and prostaglandin I2 measured as 6-keto-prostaglandin F1 alpha. Antipyrine infusion produced significant reductions in the uterine venous levels of prostaglandin E2 and prostaglandin F2 alpha metabolite (p less than 0.05). These reductions in prostaglandin levels were not associated with any significant changes in maternal blood pressure, heart rate, uterine blood flow, or oxygen content. Although fetal prostaglandin levels tended to decrease during the antipyrine infusion, these changes were not significant. Fetal blood pressure, heart rate, umbilical blood flow, PaO2, and oxygen content were not significantly altered. These data suggest that the antipyrine method does not affect basal blood flow in the uterine or umbilical circulation even though uterine prostaglandin levels are significantly decreased.