Abstract 1. 1. The effect of pyridoxine and fat deficiencies singly and in combination on certain enzyme systems in rat livers has been investigated 2. 2. Succinic dehydrogenase was unaffected in pyridoxine deficiency. In fat deficiency and in the combined deficiency of the two nutrients, its activity in liver was significantly reduced. 3. 3. Glutamic dehydrogenase in liver was significantly reduced in activity in pyridoxine and fat deficiencies. When a deficiency of fat was superimposed on that of pyridoxine a further reduction in activity was observed. 4. 4. Butyric dehydrogenase in liver was significantly reduced in fat deficiency, it was, however, unaffected in pyridoxine deficiency. No further reduction in activity was observed even when pyridoxine and fat deficiencies were coexistent. 5. 5. A reduction in the iodine value of crude fatty acids (CFA) of liver was observed in fat and pyridoxine deficiencies. A direct correlation was observed between the iodine value of liver CFA and glutamic and butyric dehydrogenase in liver.