Right and left ventricular outputs were measured continuously in fetal lambs in utero for 2 to 21 days after implantation of electromagnetic flow tranducers around the main pulmonary trunk and ascending aorta. Surface pacing electrodes were implanted on the left and right atria. The intact cervical vagus nerve was stimulated by electrodes implanted around the nerve. Spontaneous increased in fetal heart rate were associated with increases in both right and left ventricular outputs and spontaneous falls in heart rate with reduction in both outputs. There was a linear relationship between fetal heart rate and ventricular outputs. Vagal stimulation reduced fetal heart rate with a consistent and linear fall in ventricular output. Left atrial pacing increased right ventricular output by 15 to 20 per cent but produced a variable response, ususally a fall, in left ventricular output. Right atrial pacing resulted in increases in both outputs but the increase in right ventricular output was less than during left atrial pacing at the same rate. The difference in response of ventricular outputs to induced as compared to spontaneous heart rate changes relates to the effects on left and right atrial phasic pressure relationships. These changes probably alter blood flow patterns across the foramen ovale.