To test the hypothesis that coronary flow and coronary flow reserve are developmentally regulated, we used fluorescent microspheres to investigate the effects of acute (6 hr) pulmonary artery banding on baseline and adenosine enhanced right (RV) and left ventricle (LV) blood flow in two groups of twin ovine fetuses (100 and 128 d gestation, term 145 d; n = 6 for each group). Within each group, one fetus underwent pulmonary artery banding (PAB) to constrict the main pulmonary artery diameter by 50%, the other twin served as a non-banded control. Physiological measurements were made 6 h after completing surgery; tissues were then harvested for analysis of selected genes that may be involved in the early phase of coronary vascular remodeling. Within each age group, arterial blood gas values, heart rate and mean arterial blood pressure were similar between control and PAB fetuses. Baseline endocardial blood flow in both ventricles was greater in 100 d than 128 d fetuses (RV: 341 ± 20 vs. 230 ± 17, LV: 258 ±18 vs. 172 ± 23 ml/min/100 g, both p < 0.05). In both age groups, RV and LV endocardial blood flows increased significantly in control animals during adenosine infusion, and were greater in PAB compared to control fetuses. After PAB, adenosine further increased RV blood flow in 128 d fetuses (416 ± 30 to 598 ± 33 ml/min/100 g, p < 0.05) but did not enhance blood flow in 100 d animals (490 ± 59 to 545 ± 42 ml/min/100 g, p > 0.2). RV vascular endothelial growth factor and Flk-1 mRNA levels were increased relative to control (p < 0.05) in 128 d but not 100 d PAB fetuses. We conclude that in the ovine fetus, developmentally related differences exist in 1) baseline myocardial blood flows 2) the adaptive response of myocardial blood flow to acute systolic pressure load and 3) the responses of selected genes involved in vasculogenesis to increased load in the fetal myocardium.