Vascular density of the yolk sac membrane (YSM), protein and mRNA expression of hypoxia-inducible factor 2alpha (HIF-2α), vascular endothelial growth factor (VEGF-A) and its type 2 receptor (FLK-1) in the YSM were compared in subjects incubated at two different altitudes: 355 and 1378 metres above sea level (masl) in Colombia. On days 3 or 4 of incubation, higher vascular density of YSMs (p<0.05 both of the days), relative expression of HIF-2α mRNA (p<0.01 and p<0.001 days 3 and 4, respectively), VEGF-A mRNA (p<0.01 and p<0.001 days 3 and 4, respectively) and FLK-1 mRNA (p<0.001 and p<0.0001 day 3 and 4, respectively) were encountered in embryos incubated at 1378 masl as compared to those incubated at 355 masl. Information is presented about immunoexpression of HIF-2α, VEGF-A and its receptor FLK-1 in the endoderm, mesoderm and ectoderm in the YSM on days 3 and 4, and it is demonstrated that endothelial cells in the YSM on day 3 of incubation express HIF-2α, which had not been previously reported.