Plasma EPO concentration (mean ± SE) in each protocol. Protocol C-A, placebo and 8 h of air breathing; Protocol C-H, placebo and 8 h of isocapnic hypoxia; Protocol D-A, dexamethasone and 8 h of air breathing; and Protocol D-H, dexamethasone and 8 h of isocapnic hypoxia. AM, morning; PM, afternoon. Dexamethasone significantly decreased plasma EPO concentrations, whereas sustained hypoxia significantly increased plasma EPO concentrations. Dexamethasone also abolished the increase in plasma EPO concentration associated with sustained hypoxia. **P < 0.01 compared with the corresponding AM values; +P < 0.05, ++P < 0.01 compared with the PM values in Protocol C-A.