Perception of hunger and satiety was examined by means of event-related potential (ERP) recordings. On the basis of Helson's frame-of-reference theory, it was hypothesized that hunger is perceived with reference to an actual adaptation level. ERPs were recorded to adjectives (hungry, thirsty, tired) combined with one of five adverbs (very, almost, somewhat, hardly, not). Subjects indicated whether a particular adverb-adjective combination accurately described their actual feeling of hunger, thirst, or fatigue. They were tested after they had fasted for 16 hr and another time when they were satiated. Area of P300 of the ERP was smallest for adverb-adjective combinations agreeing with the subject's actual state of hunger (i.e., adaptation level), which was almost hungry in fasting subjects and hardly hungry in satiated subjects. These P300 minima were flanked by significantly enhanced (p < 0.001) P300 areas for the immediately adjacent adverbs combined with hungry. P300 areas for adverbs combined with thirsty and tired did not depend on the subject's state of hunger. The data suggest that perception of internal states, such as hunger, refers to an adaptation level that is sensitively indicated by P300 area.