The maximal myocardial blood flow (mbf) response to adenosine is a measure of the total cross-sectional area of the coronary resistance vessels. The difference between resting and maximal mbf at a given perfusion pressure defines coronary reserve. During acute fetal hypoxia and during chronic anemia mbf increases 4 fold. It is not known however if fetal coronary reserve is preserved in chronically anemic fetuses. We therefore measured mbf with microspheres and Doppler on the proximal left circumflex coronary artery in fetal sheep before and after chronic anemia induced by isovolemic hemorrhage. Aortic and vena caval occlusion was used to vary coronary arterial perfusion pressure as shown in the fig. from 1 fetus. In control fetuses (hct 30±3.0) at an interpolated mean arterial pressure of 42 mmHg, resting left ventricular mbf increased from 305±19 at rest to 664±165 ml/min/100g heart during adenosine infusion. In anemic fetuses (hct 13±0.8) adenosine increased left ventricular mbf from 892±315 to 1691±744 ml/min/100g. This suggests that the chronically anemic fetus is able to maintain coronary reserve, perhaps due to new vessel growth.