The ability to diagnose fetal cardiovascular malformations prenatally has been established during the past 14 years. This has naturally led to an increased interest in the possibility of providing timely therapy in the neonatal period and has raised the prospects for prenatal intervention. Before medical, catheter, and/or surgical interventions are performed, however, it is important to understand the normal and abnormal physiology of the fetal cardiovascular system. Significant insights have been gained into human fetal cardiovascular physiology utilizing data previously gleaned from fetal lamb models and correlating anatomic, pulsed Doppler, and color flow Doppler observations that have been made echocardiographically during the second and third trimesters of human fetal cardiac development. Regional blood flow distribution studies in the human have demonstrated a relative right-ventricular volume dominance in the human fetus of a somewhat lesser magnitude than previously described in the lamb. Observations of ventricular filling characteristics suggest a relative paucity of diastolic 'reserve' in the human fetal heart implying a relative sensitivity to acute volume overloading. The human fetal heart, like that of the fetal lamb, also appears to have a relatively modest amount of 'systolic reserve', making the heart particularly susceptible to acute ventricular afterload. Further studies have demonstrated the ability to recognize altered intrauterine shunt flow across the fetal ductus arteriosus and foramen ovale. As predicted in fetal lamb models, alterations in shunt flow accompany major malformation complexes including ventricular hypoplasia. Studies to date suggest that attempts at surgical intervention should be made very cautiously due to the inability to ascertain whether altered shunt flow patterns precede (cause?) or follow the development of structural abnormalities.