Summary. We have carried out balance studies in normal dogs in order to appraise the effects of chronic hypoxemia on acid-base and electrolyte equilibrium. During the first phase of observation we produced a state of "pure" hypoxemia by reducing the oxygen concentration (utilizing nitrogen as a diluent) and by adding carbon dioxide to the environment in a concentration sufficient to keep arterial CO2 tension (Pco2) within normal limits. The data demonstrate that such a 9-day period of normocapneic hypoxemia has no effect on electrolyte excretion and is virtually without effect on plasma composition.