The mammillary bodies have long been implicated in spatial memory, and lesions of this structure in rats can impair some spatial memory tasks. The mammillary bodies, however, comprise two main nuclei that have different electrophysiological and anatomical properties. It is therefore possible that they have different functions. The present study determined whether selective lesions of one of these components, the lateral mammillary nucleus, are sufficient to induce spatial memory deficits. While selective lateral mammillary nuclei lesions induced deficits on a working memory task in the water maze, this impairment was milder and not as persistent as that seen with complete mammillary body lesions. Furthermore, lateral mammillary nuclei lesions did not impair T-maze alternation, which is sensitive to complete mammillary body lesions. From these results it appears that lesions confined to the lateral mammillary nuclei are sufficient to produce mild impairments when rapid, new spatial learning is at a premium. At the same time, the remaining mammillary nuclei also contribute to spatial learning, though this may be in a qualitatively different manner.