Tyrosine aminotransferase (TAT) induction by glucagon and dexamethasone in the liver of tumor-bearing chickens was studied and compared with induction in healthy animals. The transplantable tumor was caused by inoculation of cells from a cell line induced by MC29 avian leukosis virus. TAT was hardly detectable in tumor tissue of control and dexamethasone-treated chickens, but it was induced by glucagon to levels which were significant although very low when compared to those in host liver or the liver of non-tumor-bearing controls after glucagon treatment. Dexamethasone failed to induce TAT in host liver at 8 A.M. while it significantly indiced TAT in the normal liver at the same time of the day. Similar failure of TAT induction was not detectable when glucagon was used instead of dexamethasone. Furthermore, it was found that diurnal variations in basal and dexamethasone or glucagon-induced TAT levels are considerably mitigated in host liver as compared to those observed in the liver of healthy animals. The possible reasons for these findings are discussed.