Immediate maternal and fetal cardiovascular responses to different doses of nicotine and carbon monoxide were studied in 24 pregnant smokers. A noninvasive pulsed Doppler ultrasound technique was used for measuring fetal blood flow in the descending thoracic aorta, the intra-abdominal part of the umbilical vein, and in the umbilical artery. Maternal plasma concentrations of nicotine, carbon monoxide, and catecholamines were measured. Maternal heart rate, blood pressure, fetal heart rate, and fetal aortic and umbilical vein blood flow increased, while pulsatility indices of the fetal aortic and umbilical artery blood velocity waveforms decreased, with increasing maternal nicotine levels; all were unaffected by carbon monoxide. Catecholamine levels remained unaffected. These results seem to confirm that the maternal nicotine intake due to smoking has an immediate, dose-dependent effect on fetal blood flow.