Study objective was to assess the effects of periodic dwelling of humans in a room with normobaric hypoxic gas mixture (NHGM) on their red blood cells. Study subjects were six men aged 20 to 24 years who were placed under NHGM at 16-15% of oxygen for 4 h daily over 2 months. Hematological parameters were determined at baseline, on the next day after the experimental series, and 3 and 6 months later. All subjects showed moderate decreases in RBC count and mean volume and hematocrit and increases in the mean levels of hemoglobin in RBC. These trends persisted at delayed times. It is concluded that periodic dwelling under NHGM conditions is associated with increases in blood oxygen capacity due to generation of more functionally competent RBC upon a tendency not to compromise the rheological properties of blood.