We tested whether infusion of medium-chain triacylglycerols (MCTs) during total parenteral nutrition (TPN) enhanced macrophage response and reduced intestinal atrophy compared with long-chain triacylglycerols (LCTs). Male Sprague-Dawley rats (230-240 g) were maintained with TPN providing 16% or 48% of nonprotein energy from MCTs plus LCTs or LCTs alone or 100% of nonprotein energy from dextrose for 6 or 12 d. Body weight gain was not significantly different among groups. Serum concentrations of beta-hydroxybutyrate were greater with MCTs plus LCTs than with LCTs alone after 6 d (P < 0.05, main effect). Triacylglycerol concentrations in liver were greater with LCTs than with MCTs plus LCTs after 6 or 12 d (P < 0.05, main effect). MCTs plus LCTs increased by 50% the percentage (P < 0.0005) and number of splenic macrophages compared with LCTs alone in conjunction with decreased triacylglycerol concentrations in spleen after 6 d (P < 0.05, main effect). In vitro tumor necrosis factor alpha secretion by splenic or circulating macrophages in response to lipopolysaccharide was increased by MCTs plus LCTs compared with LCTs alone, twofold after 6 and sevenfold after 12 d (P < 0.05, main effect). Jejunal mucosal mass was 30% greater with MCTs plus LCTs than with LCTs alone after 6 or 12 d (P < 0.01); villus height was also significantly greater after 6 d (main effect). The incidence of bacterial translocation to the mesenteric lymph nodes was not significantly different among groups. Compared with LCTs, MCTs enhanced macrophage response and decreased intestinal atrophy.