The effects of nicotine on uterine and umbilical hemodynamics were studied in seven pregnant ewes equipped with electromagnetic flow probes around the main uterine and common umbilical arteries, and with polyvinyl catheters inserted into the fetal aorta and maternal femoral vessels. Maternal systemic infusions of 1.0 to 1.5 mg/min significantly decreased uterine blood flow by 42% (p less than 0.001) and 32% (p less than 0.05), respectively. However, nicotine infusion rates of 0.5 mg/min, resulting in mean peak maternal nicotine concentrations of 130 +/- 5.0 ng/ml (SE), a value substantially higher than that reported in smoking humans, did not significantly alter uterine or umbilical vascular hemodynamics. Furthermore, at this lower nicotine dose, no alterations were observed in maternal or fetal plasma norepinephrine and epinephrine. The findings suggest either a species difference between sheep and humans in the threshold of catecholamine release to nicotine exposure or the need for repetitive nicotine infusions throughout the day to stimulate the human smoking experience.