Experiments were conducted on albino rats; it was revealed that an increase in CO2 content in the inspired air (3.8%) caused disturbances in tyrosine and tryptophane metabolism. The activity of tyrosine-aminotranspherase and of tryptophane-oxygenase proved to increase in the liver; blood serum displayed a reduced concentration of free tyrosine and free total tryptophane, but the level of free tryptophane obtained by dialysis proved to rise. A possible significance of these deviations in endogenous blastomogenesis is discussed.