The objective was to determine the effects of intravenous infusion of triacylglycerol (TAG) emulsions derived from different lipid sources on energy metabolism during a 4-d fast. Six nonpregnant, nonlactating multiparous Holstein cows were randomly assigned to treatments in a replicated 3 x 3 Latin Square design. Treatments included intravenous infusion of tallow, linseed oil, or fish oil emulsions at a rate of 0.54 g of TAG/kg of body weight per day; infusions were concurrent with a 4-d fast. The emulsions were administered for 20 to 30 min every 4 h throughout the 4-d fast. Cows were fed ad libitum for 24 d between the fast/infusion periods. Infusion of tallow, linseed oil, or fish oil emulsions increased plasma concentrations of palmitic acid, linolenic acid, and eicosapentaenoic and docosahexaenoic acids, respectively. Infusion of linseed oil emulsion decreased plasma TAG concentrations compared with tallow and fish oil treatments, which were similar. Infusion of the tallow emulsion resulted in the highest concentrations of plasma nonesterified fatty acid (NEFA), insulin, and glucose, whereas the infusion derived from linseed oil had the lowest NEFA and beta-hydroxybutyric acid concentrations. The different TAG emulsions had no effect on total or peroxisomal oxidation of [1-(14C)]oleic acid in liver homogenates. Liver TAG content increased 12.0, 7.8, and 14.1 microg/microg of DNA during the fast for tallow, linseed oil, and fish oil treatments, respectively; linseed oil was different from fish oil and tended to be different from tallow.