The fetal heart beat, detected by ultrasound, was recorded for five minutes from 934 antenatal women--266 at 30 to 35 weeks' gestation and 718 at 36 to 41 weeks. Episodes of bradycardia < 100/minute ranging from 5-30 seconds' duration occurred in 3 (1.1%) of the former and in 9 (1.3%) of the latter group. Tachycardia > 180/minute ranging from 30-90 seconds' duration occurred in 1 (0.4%) of the former and in 4 (0.6%) of the latter. Premature beats were not detected between 30 and 35 weeks, but occurred in 12 (1.7%) of 720 at 36 to 41 weeks' gestation. 50 subjects were monitored at both 30 and 35 weeks and 36 to 41 weeks' gestation and in one woman premature beats were present in the latter but not in the former recording. The incidence (1.7% of premature beats in the fetus at 36 to 41 weeks' gestation was similar to that in the healthy neonate (0.8%). Recordings of arrhythmias or rates outside the range 100-180/minute were replayed through a standard antepartum fetal heart rate monitor. The monitor failed to detect premature beats, 2 to 4 episodes of tachycardia > 180/minute, and 9 of 12 episodes of bradycardia < 100/minute, stressing its unreliability for detecting rapidly changing rates.