A quantitative assessment of the affects of acute and chronic hypoxia on the oxygen transport and the distribution of systemic cardiac output is being studied in the conscious intact dog. Dogs native to Denver are studied before and after a 3 week exposure to a low oxygen environment, equivalent to 4500 m. 50 micron radioactive microspheres labelled with Sr85, Yb169 are used to study the regional distribution of blood flow. In addition, blood pressures, cardiac output, blood gases and blood saturations, hemoglobin and hematocrit are measured. This information is important in the understanding of the homeostatic mechanisms involved in adaptation to hypoxia.