Values are means +s.e.m. for chorioallantoic venous blood taken from sea level chick embryos incubated either at sea level (SLSL, open bar, n = 9) or high altitude (SLHA, filled bar, n = 12), high altitude embryos incubated at high altitude (HAHA, stippled bar, n = 10) or sea level (HASL, hatched bar, n = 7), and from sea level chick embryos incubated at high altitude with oxygen supplementation (SLHA + O2, grey bar, n = 12). Different letters are significantly different by one-way ANOVA with Student–Newman–Keuls test (P < 0.05).