Currently, renal tissue hypoxia is the only widely accepted trigger for erythropoietin (EPO) production. However, previous studies in healthy subjects have demonstrated that a sudden and sustained decrease in tissue oxygen level, aside from an absolute low level of tissue oxygen tension, could also act as a trigger for EPO production. To confirm these observations and to clarify an eventual role of free oxygen radicals and antioxidants, hypobaric pure oxygen will be administered to healthy subjects.