The effects of cytotoxic lesions in either the anterior thalamic nuclei or the mamillary bodies were compared with those of fornix lesions on a test of spatial working memory. All three lesions impaired acquisition of a forced alternation task in a T-maze, but the disruptive effects of the mamillary body lesions were significantly less than those following either fornix or anterior thalamic damage. When the alternation task was changed, so as to increase proactive interference, the impairment associated with mamillary body damage became more evident and was now equal in severity to that in the animals with anterior thalamic lesions. The fornix lesion group were the most impaired. In contrast, all three groups performed normally on a test of object recognition. The results add weight to the view that hippocampal — anterior thalamic connections are critical for normal spatial memory and that the relative contribution of the mamillary bodies is task dependent.