Increased plasma erythropoietin (ESF) activity, a rise in RBC 2,3-DPG, and a ‘right shift’ of the oxygen hemoglobin equilibrium curve following androgen administration to mice in ambient or hyperoxic conditions, was demonstrated. If androgens had a direct effect on the RBC metabolism, increased 2,3-DPG would result in a facilitated release of oxygen to the tissue. This would have been accompanied by a decrease rather than an increase in the level of ESF. Hyperoxia has abolished detectable rising levels of plasma ESF and RBC 2,3-DPG following androgen administration. These levels were close to those seen in the ambient nontreated mice.