EPO and chronic mountain sickness: 2004). The values throughout 24 h varied between approximately 96 and 91%. According to Weil et al. (1968), the threshold for the stimulation of Epo secretion is located within this range of arterial saturations. Because of the “shoulder” of the ODC, equal diurnal differences in arterial PO2 cause larger SaO2 variations at altitude than at sea level. When the arterial PO2 decreases more markedly in chronic mountain sickness, [Epo] fluctuations are exaggerated, but a diurnal rhythm ceases to exist (Bernardi et al. 2003).