A laboratory study of the aggressive behaviour of the vole (Microtus agrestis) has been made as part of an investigation of the consequences of CHITTY's hypothesis concerning the cause of vole population cycles. Adult male voles will attack other voles, whether male or female. Some males are more successful in fights than others and they come to be the dominants of the groups to which they belong. Females in advanced pregnancy or those nursing a litter will attack any animals which come near the nest, including dominants that had formerly chased them. Before a male attacks another vole he may move in a jerky fashion through the cage. The jerky movements possibly indicate conflict between aggression and flight. Or an aggressively motivated male may stay in one small area of a cage with his body hunched and with hair erected. While staying at the same spot his legs may make walking movements (marking time), he may turn round on the spot (waltzing), or still retaining the hunched posture and erected hair, he may travel with frequent changes of direction and with short, rapid, leg movements through part of the cage (dancing). This behaviour seems also to indicate activation of both the aggressive and the flight drives, the walking movements being part of the appetitive behaviour of both drives. Waltzing and the frequent changes in direction of movement in dancing, can be regarded as representing the taxis component of attack (moving towards an opponent) and of escape (moving away from an opponent). If an aggressive vole succeeds in catching a subordinate, he may settle upon him and inflict severe wounds. However, a subordinate, before being caught, may retaliate by turning and facing the pursuer. The subordinate then lunges with his incisors bared at the oncoming aggressive animal, or, squatting on his hindquarters, squeals loudly each time the aggressive animal comes near. The retaliation of a subordinate often causes a dominant to retire. Occasionally a subordinate vole that is being chased stops suddenly with his tail erected sharply. The dominant may still be quite near, yet there will not be an attack. This posture of the subordinate may subserve appeasement, as may also the supine posture sometimes assumed by subordinates immediately in front of dominants. When a dominant retires from a retaliating subordinate, he may go to another part of the cage and dig in the sawdust with his fore and hindlegs. This behaviour seems to be a displacement activity, the digging being autochthonously employed in food seeking, tunnel construction and defaecation or urination. The displacement digging is probably caused by the activation of the flight drive in an animal whose aggressive drive is highly activated. Following such digging activity, the dominant animal may re-approach the subordinate inducing it to flee, or the subordinate's repeated retaliation may once more cause the dominant to retire and to do some further displacement digging. In between the successive approaches of an aggressive dominant to a retaliating subordinate, the subordinate may briefly brush its nose with its fore paws. This also seems to be a displacement activity, autochthonously forming part of toileting. Displacement toilet can be interpreted as being caused by slight activation of the aggressive drive of an animal whose flight drive is highly activated.