The coronary circulation is the major determinant of myocardial oxygen balance which in turn is necessary for adequate cardiac function under a variety of conditions. Experiments in the fetal lamb suggest short-term functional coronary autoregulation in response to increase in afterload and acute hypoxemia. Long-term, acute-on-chronic hypoxemia is associated with a marked increase of maximal myocardial flow reserve suggesting coronary vascular angio-neogenesis during the period of chronic hypoxemia. In the human fetus, flow velocity waveforms from the coronary arteries may be obtained by color-coded and pulsed wave Doppler sonography in normally developed fetuses from 31 weeks of gestation onwards under favorable imaging conditions. Coronary blood flow may also be visualized in selected fetuses with intrauterine growth restriction, absent or reversed end-diastolic flow in the umbilical artery, cephalization of blood flow and abnormal flow patterns in the precordial veins and the umbilical vein. In these fetuses visualization of coronary blood flow may be possible as early as 26 weeks gestation and is consistently associated with a significant increase in the peak velocity index for veins in the ductus venosus. In this circulatory state, visualization of coronary blood flow is suddenly very easy. This "visualization threshold" may be a sign of maximal increase in coronary blood flow in an attempt to "spare" the fetal heart of hypoxemia. Although highly operator dependent, demonstration of coronary blood flow in intrauterine growth restriction identifies fetuses at high risk for intrauterine fetal death and postpartum circulatory failure.