The mechanism involved in the increase of lymph alkaline phosphate after fat feeding has been studied utilizing male rats with mesentric lymph fistulas. There was good correlation between the levels of alkaline phosphatase and triglycerides in mesentric lymph both in the basal state and after administration of micellar solutions of oleic acid. Increased phosphatase activity in lymph occurred within the first hour after fate feeding, and this response was not altered in animals with a bile fistula. After the infusion of micellar solutions of octanoic acid, a medium chain fatty acid absorbed via the portal stream, there was a rise in lymph alkaline phosphatese but without an elevation of triglyceride. The suggests that the stimulus for the elevation of phosphatase in the lymph during lipid absorption involves the uptake or transport of lipid into the cell rather than its exit into the lymph. Acetoxycycloheximide, an inhibitor of protein synthesis, prevented the rise in lymph alkalinee phosphatase after infusion of micellr solutions of both oleic and octanoic acid suggesting that increased synthesisof the enzyme was involved. In acetoxycyoloheximide treated male rats receiving micellar solutions of oleic acid there was still some inrease in lymph triglyceride although there was no elevation of lymph alkaline phosphatase. Therefore alkaline phosphatases is probably not essential for the transport of lipid into lymph.