Apolipoprotein AIV (apoAIV) is a lipid-binding protein that is secreted by the small intestine in response to dietary lipid. In vitro evidence suggests that over-expression of apoAIV may protect against atherosclerosis because of changes in reverse cholesterol transport or the oxidation of plasma lipids. However, it is unclear how the knock-out of apoAIV so drastically reduces blood lipids. This important role of apoAIV in lipid metabolism is as yet undefined. The apoAIV KO mouse is a convenient and well- established model for addressing this intriguing question. Based on our preliminary data in apoAIV KO mice, we hypothesize (1) that apoAIV plays an important physiological role in mediating the efficiency with which chylomicrons are taken up by the peripheral tissues and liver; 2) that apoAIV acts in the liver to impact the synthesis and/or secretion of TG and cholesterol; and 3) that loss of circulating apoAIV is protective against hyperlipidemia caused by chronic feeding of a high-fat diet.