Erythropoietin (EPO) preserves arterial oxygen content by controlling red blood cell and plasma volumes. Synthesis of EPO was long thought to relate inversely to renal oxygenation, but in knockout mice, brain and skin have been identified as essential for the acute hypoxic EPO response. Whether these findings apply to humans remains unknown. We exposed healthy young subjects to hypoxia (equivalent to 3800 m) and measured EPO in arterial and jugular venous plasma and in cerebrospinal fluid. To examine the role of the skin for EPO production during hypoxia, subjects were exposed to 8 h of hypobaric hypoxia with or without breathing oxygen-enriched air to ensure systemic normoxemia. With 9 h of hypoxia, arterial EPO increased (from 6.0±2.2 to 22.0±6.0 mU/ml, n=11, P