Gastric intubation of young male rats with 100 kJ/d of medium chain (C8 and C10) triglyceride (MCT) reduced their voluntary intake of stock diet such that total metabolizable energy intake was similar to that of rats intubated with water, and 41% of their energy intake was derived from MCT. Body weight, energy gain, and energetic efficiency were all markedly suppressed in MCT-fed rats, but energy expenditure over the 14-day experiment was significantly increased. Resting oxygen consumption, measured at thermoneutrality, was also enhanced in MCT-fed rats, but this difference was abolished by injection of the animals with the beta-adrenergic antagonist, propranolol. Brown adipose tissue mass was similar for both groups, but the activity of the mitochondrial proton conductance pathway, assessed from the binding of purine nucleotides, was increased by over 70%. These data indicate that the reduced weight gains of animals fed MCT are due to elevated rates of energy expenditure, possibly resulting from sympathetic activation of brown fat thermogenesis.