In 1300 Doppler assessments performed on the fetal cerebral arteries, 5 cases of heart rate deceleration were recorded while studying the middle cerebral artery. There were 3 normal fetuses (mean menstrual age ± standard deviation: 28.6 ± 2.1 weeks), one growth-retarded fetus (34 weeks, menstrual age) and 1 fetus with red cell alloimmunization (27 weeks, menstrual age). The magnitude of decline in the heart rate during the deceleration ranged from 44 to 93 bpm (67 ± 18 bpm). Pulsatility index values were calculated on 44 waveforms and correlated with the heart rate. Linear regression analysis showed a statistically significant inverse correlation between the heart rate and the pulsatility index in the middle cerebral artery of 4 fetuses. These results indicate that heart rate will affect the pulsatility index of the middle cerebral artery.