Cristancho, Edgar, Orlando Reyes, Mauricio Serrato, Maria Mercedes Mora, Joel Alberto Rojas, Yohan Robinson, and Dieter Böning. Arterial oxygen saturation and hemoglobin mass in postmenopausal untrained and trained altitude residents.—High Alt. Med. Biol. 8:296–306, 2007.—Because of lacking ventilatory stimulation by sex hormones in postmenopausal women (PW), one might expect a lowered arterial oxygen saturation (SO2) in hypoxia and therefore a stronger erythropoietic reaction than in young women (YW). Nine untrained (UTRPW) and 11 trained (TRPW) postmenopausal altitude residents (2600 m) were compared to 16 untrained (UTRYW) and 16 trained young women (TRYW) to check this hypothesis and to study the combined response to hypoxia and training. SO2 was decreased in PW (89.2% ± 2.2 vs. 93.6 ± 0.7% in YW, p < 0.01). Hb mass, however, was similar in UT (UTRYW: 9.2 ± 0.9 g/kg−1, UTRPW: 8.7 ± 1.0 g/kg). But if body fat rise with age was excluded by relation to fat-free mass, Hb mass was increased in UTRPW (+1.2 g/kg, p < 0.05) compared to UTRYW. Training caused a similar rise of Hb mass in PW and YW (0.3 g/kg per mL/kg · min−1 rise in O2peak). There was no difference in erythropoietin among the groups. Ferritin was higher in PW than YW. The results show that female hormones and fitness level have to be considered in studies on erythropoiesis at altitude. The role of erythropoietin during chronic hypoxia still has to be clarified.