We compared, in six subjects, the thermic effect of food (TEF) after the ingestion of a test meal with that observed after the delivery of an equivalent test meal directly into the stomach through a nasogastric tube. TEF was measured after each test meal (i.e., ingested or tube delivered) until postprandial metabolic rate was not different from fasting metabolic rate (as measured at approximately the same time of day on a previous day). TEF after the tube-delivered meal was not significantly different in magnitude or duration from TEF after the ingested meal. The two types of meals also resulted in similar changes in respiratory quotient. These results suggest that the majority of TEF arises after the food reaches the stomach and that very little of TEF is produced by sensory factors or by the mechanical aspects of eating.