The in vivo effects of increased delivery of fatty acids (FA) to the liver are poorly defined. Therefore, we compared the effects of infusing either 6 mM oleic acid (OA) bound to albumin, 0.5-20% Intralipid, or saline for 3 or 6 h into male C57BL/6J mice. Infusions were followed by studies of triglyceride (TG) and apoB secretion. Although plasma FA levels increased similarly after either 20% Intralipid or 6 mM OA, TG secretion increased only after infusion of 4 -20% Intralipid; TG secretion was unchanged by 6 mM OA. By contrast, 6-h infusions of either 6 mM OA or 4 -20% Intralipid increased apoB secretion. 6 mM OA and 20% Intralipid each increased secretion of apoB from primary hepatocytes ex vivo. Importantly, 0.5-2% Intralipid, which delivered more FA to the liver than 6 mM OA, did not stimulate apoB secretion. Hepatic apoB mRNA levels were unaffected by either 6 mM OA or 20% Intralipid, but microsomal triglyceride transfer protein mRNA was significantly lower after 6-h infusions with 6 mM OA versus either saline or 20% Intralipid. Lower microsomal triglyceride transfer protein mRNA levels were associated with reduced hepatic TG mass after 6-h infusions of 6 mM OA. We conclude that 1) increased FA delivery to the liver in vivo increases secretion of apoBlipoproteins via post-transcriptional mechanisms, 2) OA-induced apoB-lipoprotein secretion occurred at least in part via mechanisms other than by providing substrate for TG synthesis, and 3) the route of delivery of FA is important for its effects on apoB secretion.Regulation of the assembly and secretion of apolipoprotein B (apoB)-lipoproteins and, in particular, very low density lipoprotein (VLDL), 1 by the liver has been studied intensively for many years in cultured hepatocytes (1-4). Many laboratories have also conducted in vivo studies of apoB-lipoprotein secretion in humans and animal models (5-7). However, despite the presence of a large body of work, several questions remain incompletely answered.First and foremost is the question of whether increased flux of plasma fatty acids (FA) to the liver can stimulate VLDL secretion. Studies in cultured liver cell lines, primary hepatocytes, and perfused livers have provided conflicting results relative to the role of FA delivery in VLDL secretion. Thus, several (8 -13), but not all (14 -16) studies in cultured liver cell lines support the proposal that apoB secretion is increased by increased FA uptake. Studies in primary hepatocytes from fed rats (16 -18) or hamsters (19,20) have failed to show that exogenous FA stimulate secretion of apoB. However, we were able recently to demonstrate oleic acid (OA)-induced apoB secretion in primary hepatocytes from fasted mice (21). Results from perfused rat livers have also been mixed; OA had no effect on apoB secretion in chow-fed rats (22) but increased apoB secretion in fasted (22) or high carbohydrate diet-fed (23) rats. In vivo studies in humans have also been inconclusive. Lewis et al. (24,25) demonstrate that, when they increased the plasma levels of FA by infusin...