This study was carried out with three groups of weanling rats. One group was fed a high-protein (20%) diet, another group a low-protein (2.5%) diet, the third group a high-protein diet in restricted amounts. After 4 weeks of feeding, rats were injected simultaneously with L-[G-3H]-tryptophan and [carboxyl-14C]-nicotinic acid. The ratio of incorporation of [3H]-tryptophan to that of [14C]-nicotinic acid into liver NAD and NADP was found to be higher in protein-restricted rats. On the other hand, the ratio was found to be reduced in diet-restricted group of rats compared with ad libitum fed or low-protein diet fed groups. These results suggest that the efficiency of conversion of tryptophan to NAD is increased in protein deficiency, but reduced in the diet restriction. These observations are in line with our earlier findings on the changes in liver quinolinate phosphoribosyltransferase (EC 2.4.2.19) activity following feeding of low-protein or restricted diets. It is suggested that this technique of measuring the incorporation of two isotopes from the substrates labelled with two different isotopes can be conveniently used as a tool to measure the relative contribution of tryptophan and nicotinic acid to the synthesis of nicotinamide nucleotides.