1. Dietary tryptophan was found to regulate the activities of tryptophan oxygenase (EC I. 13.I. 12)and quinolinate phosphoribosyltransferase (EC z.4. 2a) in liver.
2. With increasing tryptophan concentration in the diet containing 100 g protein/kg, tryptophan oxygenase activity increased while that of quinolinate phosphoribosyltransferase decreased. The response of these enzymes to dietary tryptophan at lower dietary protein level (25 g/kg) was not significant.
3. Liver nicotinate phosphoribosyltransferase (EC 2.4.2. 11)activity and kidney picolinate carboxylase (EC 4. I.I. 45) activity were unaltered with different tryptophan concentrations in the diet.
4. The response of various biochemical measurements was dependent on the tryptophan intake and the changes were marked below and above the requirement level of tryptophan.
5. It is suggested that the urinary excretion of quinolinic acid and N'-methylnicotinamide may be useful in assessing the tryptophan nutritional status and its requirement.