Following digestion of dietary triacylglycerol (TAG), intestinal epithelial cells absorb fatty acids and monoacylglycerols that are resynthesized into TAG by enzymes located on the endoplasmic reticulum (ER).The intestinal epithelial absorptive cell has little control over the rate of entry of the absorbed products of lipid digestion, fatty acids (FAs) and monoacylglycerols (MAGs). These must be disposed of rapidly, otherwise the cells risk destruction of their cellular membranes. The enterocytes are able to store some of the FA (and MAG) bound to liver FA binding protein (L-FABP), which is expressed in the intestine (1). Most of the remainder is rapidly resynthesized into triacylglycerol (TAG) (2). The synthetic steps take place on the membrane of the endoplasmic reticulum (ER), which is the site of the neutral lipid acyl transferases. Exactly on which face of the ER membrane the last enzyme in this pathway, diacylglycerol acyltransferase (DGAT) [which acylates diacylglycerol (DAG) to TAG], exists is controversial.A pathway for the synthesis of TAG within the lumen of the ER of liver cells, presumably leading to the synthesis of VLDL particles, has been identified that utilizes cytosolic TAG and acyl-CoA to produce TAG within the ER lumen. The ER lumenal TAG was found to consist of two acyl groups from the cytosolic TAG and one acyl group from cytosolic FA-CoA (3). This pathway of intralumenal ER synthesis of TAG was shown to be carnitine dependent and was inhibited by glybenclamide, a potent inhibitor of carnitine acyltransferase (3). These investigators predicted the existence of a lipase outside the ER and a second DGAT within the hepatic ER lumen. A lipase is known to exist in intestinal cytosol (4), and two DGATs have recently been identified in the intestine by Farese's laboratory (5, 6).Studies in liver by Coleman and Bell (7,8) showed that DGAT was on the cytosolic face of the ER, as suggested by protease digestion experiments with and without ER membrane permeation. These studies were later challenged by Saggerson and by Zammit, who found that only on liver ER membrane disruption could the full activity of DGAT be expressed (9-11), suggesting that DGAT was in part a latent enzyme; that is, in order for full activity to be