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 apoB-lipoproteins 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.