The weight of 6- and 7-d-old chicken embryos and their chorioallantoic membrane (CAM), vascular density, protein immunoexpression, and mRNA of hypoxia inducible factor 2 (HIF-2α), vascular endothelial growth factor (VEGF-A), and its type receptor 2 (FLK-1) in the CAM were compared in subjects incubated at 2 different altitudes: 355 and 1,378 m above sea level (masl) in Colombia. A difference was not found when comparing the weight of CAM and embryos incubated at 355 masl with those incubated at 1,378 masl on d 6 and 7 of incubation (P > 0.05). Higher vascular density in CAM (P < 0.05), percentage of cells immunoexpressing HIF-2α and FLK-1 in the CAM (P < 0.05), and relative expression of HIF-2α and FLK-1 mRNA in the CAM (P < 0.05 and P < 0.01, respectively) were encountered in embryos incubated at 1,378 masl compared with those incubated at 355 masl, but only on d 6. Percentages of cells immunoexpressing VEGF-A and relative expression of VEGF-A mRNA in the CAM values were not different when considering altitude and age (P > 0.05). Relative hypoxia (1,378 masl) appears to affect HIF-2α and FLK-1 expression in CAM of 6-d-old chicken embryos, and this change in expression results in increased vascular development in CAM on this day. At hatching, chickens between treatments differed in their BW (P < 0.0001). The lack of differences in findings on d 7 could be due to morphological, physiological, and molecular events occurring at this time.