Thirty healthy pregnant women were studied to assess the immediate cardiovascular responses of the fetus to the smoking of one cigarette. The fetal blood flow was measured in the aorta and in the umbilical vein by combining real-time ultrasonography and the pulsed Doppler technique. Following maternal smoking, a significant increase was found in the maternal heart rate and also in the blood pressure. In the fetus, a significant transient increase in the aortic and the umbilical blood flow was measured, as characterized by the increase in the fetal heart rate, the mean and maximum blood velocity, and the vessel diameter. Thus, maternal smoking induced acute circulatory changes in the fetus similar to those found in adults. Furthermore, the study demonstrated the feasibility of the method to evaluate non-invasively the immediate effect of a given stress stimulus on the cardiovascular system of the human fetus.