Plasma levels of glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), lactic dehydrogenase (LD), phosphohexose isomerase (PHI), aldolase (Aid), isocitric dehydrogenase (ICD), malic dehydrogenase (MD), and glutathione reductase (GR) were studied serially in hamsters for 30 days after transplantation of a non-metastasizing mammary adenocarcinoma. The transaminase (GOT and GPT) levels remained essentially normal throughout the studies. The highest levels of the other enzymes were observed between the 22d and the 30th days. Values for the PHI and ICD showed the greatest degree of elevation, levels of the Aid and MD somewhat less marked elevations, while values of the LD and GR were least elevated. The highest plasma-enzyme levels were found when tumor mass was greatest, but tissue necrosis was also prominent. Nevertheless, distinctly elevated levels appeared even when necrosis was negligible.