To study the possible participation of food-induced sensory stimulation on meal thermogenesis an experiment was performed with eight female subjects. On alternate days subjects were fed either a highly palatable meal (HPM), containing 710 calories, or a nonpalatable meal (NPM). The NPM was prepared by mixing all the ingredients of the HPM and was presented to the subjects as a desiccated biscuit. The subjects were not informed about the composition of the NPM, which they rated as tasteless and unappetizing. The increase in O2 consumption was approximately 20% during the 90 min following the HPM compared with 12% with the NPM (P less than 0.01). With comparable increases in plasma glucose, plasma insulin level was significantly (P less than 0.01) lower following NPM ingestion than with ingestion of the HPM. At that time a significant increase in plasma norepinephrine was also observed but only following ingestion of the HPM. It would appear that both central sensory stimulation or plasma insulin level, as affected by food palatability, could be considered at this time as possible activators of the increased sympathetic activity observed following ingestion of the HPM. It is suggested that a part of meal thermogenesis is due to food palatability and that the concomitant activation of the sympathetic system may be related to this action.