Hypertriglyceridemic subjects were fed diets in which dietary fat calories were held constant, but carbohydrate calories were varied. Three subjects with fasting chylomicronemia (Type V) were given less carbohydrate and four subjects without fasting chylomicronemia (Type IV) were fed diets with more calories as carbohydrate. The restricted carbohydrate intake led to disappearance of chylomicronemia in those subjects who had chylomicronemia on a normal diet (Type V to IV). In those subjects without chylomicronemia, chylomicronemia appeared in response to increased carbohydrate intake (Type IV to V). Thus chylomicron concentrations in plasma were altered even though fat intake and presumably chylomicron input into plasma was kept constant. These findings provide evidence for saturation of chylomicron removal mechanisms by alteration of endogenous triglyceride-rich lipoprotein concentrations. They suggest that chylomicrons compete with very low density lipoproteins for similar removal mechanisms. The relationship between endogenous triglyceride concentration and the lipolytic activity in plasma following heparin was then evaluated with the use of long-term heparin infusions to release and maintain lipolytic activity in the circulation. 10 subjects were placed on fatfree diets to remove circulating dietary fat. The plasma lipolytic rate during the heparin infusion was measured consecutively on different days in individuals whose triglyceride concentrations were varied by either increasing or decreasing calories. The lipolytic rate was curvilinearly related to the plasma triglyceride concentrations. This curvilinear relationship followed Michaelis-Menton saturation kinetics over a wide range of triglyceride concentrations on fat-free, high-carbohydrate diets, in multiple studies in a group of individuals. These studies suggest that endogenous and exogenous triglyceride compete for a common, saturable, plasma triglyceride removal system related to lipoprotein lipase.