With an improved method of solubilization and assay, glutamate dehydrogenase (EC 1.4.1.2) was quantified in a series of normal and neoplastic rat tissues. Fetal and neoplastic tissues had generally low activities but in mammary tumors and rhabdomyosarcoma the activity exceeded that of their cognate tissues. In liver, glutamate dehydrogenase rose rapidly during the last days of gestation and reached its adult level immediately after birth.