As previously observed in vivo, statistically significant variations characterized by increase in chlorine and diminution in sodium, associated with an increase in red cell volume, have been noted in vitro in samples of venous blood taken from patients undergoing tonometry under oxygen. These variations appear to be independent of the basic concentration of chlorine and do not occur if the blood samples are incubated with theophylline at the same time. These results confirm the hypothesis of a "non respiratory" plasmatic acidifying effect of oxygen therapy inhibited by the antiphosphodiesterasic action of theophylline.