The effect(s) of dietary pyridoxine availability on serine dehydratase (SD) specific activity levels of normal liver, Morris hepatomas #5123A, 7316B, 7800, and of respective host livers was studied. Buffalo female weanling rats were fed ad libitum a pyridoxine-free diet or the same diet supplemented with the vitamin. They were inoculated intramuscularly in the hind leg muscles with hepatoma cells after 3 weeks on the respective diets, and those bearing hepatomas #5123A, 7316B, 7800 were killed at 28,30, and 48 days, respectively, after inoculation. SD activity was highly affected by pyridoxine. Absence of the vitamin from the diet resulted in greatly reduced activity levels in normal liver and the three hepatomas. Tumors grown in animals fed the pyridoxine-supplemented diet had 3.4 (#5123A), 3.5 (#7316B), and 2.1 (#7800) times more SD specific activity than respective tumors grown in animals fed the deficient diet. A 1.7-fold increase was observed in normal liver. In contrast to these findings, the specific activity of the enzyme was reduced by 6.3, 1.5, and 3.0 times, respectively, in the host livers of animals fed the vitamin-supplemented diet and bearing hepatomas #5123A, 7316B, and 7800. Serine dehydratase activity depends greatly on dietary vitamin B6 and hence I propose that activity levels in vivo are regulated by its presence or absence.