Nicotinate phosphoribosyltransferase (NPRTase, EC 2.4.2.11) purified from hog liver was stimulated by orthophosphate. This stimulation was independent of ATP which activated this enzyme by increasing the velocity and lowering the Km values for nicotinic acid and 5-phosphoribosyl-1-pyrophosphate (PRPP). This independent stimulation by orthophosphate and activation by ATP were true of hog liver or rat liver homogenate as well as the purified hog liver NPRTase, showing that there may be some physiological role of these compounds in vivo. The Ka value for ATP was about 1.2mM. At a physiological concentration of PRPP, it was mainly used by quinolinate phosphoribosyltransferase (QPRTase, EC 2.4.2.19) in the reconstituted assay conditions of NPRTase and QPRTase if the ATP concentration was lower. But if the ATP concentration was higher, PRPP was used by both QPRTase and NPRTase. Therefore, ATP was found to play a very important role for the biosynthesis of NAD via the salvage pathway.