Early in the course of experimental pneumococcal infection in mice, a significant induction of the liver enzyme, tryptophan pyrrolase, occurs. The importance of endogenous adrenal secretions in producing enzyme induction was demonstrated. Changes in total liver protein synthesis during infection may parallel to a degree changes in tryptophan pyrrolase production and are also based on normal adrenal function. Alterations in these two apparently related phenomena indicate that a protein anabolic phase occurs during the course of experimental pneumococcal infection. The importance of these changes is discussed in terms of normal host defense mechanisms.