To evaluate the relationship between heart rate and rhythm, and cardiac performance, in the human fetus in utero, observed over a long-term period in gestation, we made a total of 138 studies in 114 fetuses from 18 to 41 weeks of gestation; 104 having heart rate changes without rhythm disturbances ("control group"), 6 with complete atrioventricular block and 4 with supraventricular tachycardia. Using M-mode echocardiogram, we measured end-diastolic dimension and fractional shortening in the right and left ventricles. The corresponding heart rate for each cardiac cycle was measured using the interval between two consecutive end-systolic points. In fetuses in the group of controls, the values for fractional shortening in both ventricles were almost constant with advancing gestational age, unrelated to an increase in end-diastolic dimensions of either ventricle. There was no correlation between changes in heart rate and changes in the value of ventricular fractional shortening at any period of gestation studied. In the fetuses with atrioventricular block, dimensions and fractional shortenings in both ventricles were significantly larger than those in the group of control fetuses at the same stage of gestation. This indicates that the fetal heart is capable of acclimatizing itself, beginning as early as 18-25 weeks of gestation, to long-lasting bradycardia in which an increased stroke volume would be required. In fetuses with supraventricular tachycardia, end-diastolic dimensions were larger and fractional shortening was significantly smaller in both ventricles than in fetuses from the control group from 26-30 weeks of gestation onwards. This suggests tachycardia-induced cardiomyopathy occurring during intrauterine life.