In this paper the abnormal behavior induced by long-term competition for food between two rats cohabitating in the same operant chamber is discussed with regard to its application to the animal model of psychopathology and with reference to the Hebb's criteria of neurosis. Two male rats which had learned individually to press a lever and to receive a pellet of food under a fixed ratio schedule were put into an operant chamber with two levers and one feeder. The pair of rats, successively exposed to three conditions, showed various behavioral patterns on the basis of the dominance hierarchy as follows: (1) With the competition for food under free feeding condition, only the subordinate rat shwed abnormal behavior, i. e. heterophagia. (2) Under the restricted feeing condition, the subordinate rat's abnormality increased and, concomitantly, circadian rhythms of various activites shifted to feeding time except for aggressive activity. (3) With return to individual housing and free feeding, only the subordinate rat showed this abnormality as the disruption of rhythms for at least one week. Since this abnormal behavior corresponds to Hebb's definitions, the abnormal behavior of this model suggests the possibility of its application to experimental neurosis.