The present study examined the effects of lesions of the mammillary system on spatial memory and arousal. Destruction of the medial mammillary nucleus or the mammillotegmental tract produces impairments on a delayed alternation task and greatly increases activity in the open-field. Lesions of the mammillothalamic tract produce a differential effect in that the spatial alternation deficit is accompanied by a general lethargy and unresponsiveness. It is suggested that the mammillary system plays a crucial role in the short-term storage of proprioceptive information necessary for the successive execution of maze choices. It also appears to play a role in the arousal state of the animal.