A disconnection analysis determined the extent to which the fornix, hippocampus and entorhinal cortex are components of the same functional system in tasks that require working memory. Preoperatively, rats were trained to perform accurately on a radial arm maze. Then various combinations of unilateral and bilateral lesions were placed in the fornix and entorhinal cortex, either with or without a transection of the hippocampal commissures. When the lesions left intact at least one pathway through the hippocampus interconnecting the fornix and entorhinal cortex, rats performed normally. Either an uncrossed pathway (following a unilateral lesion of the fornix, transection of the hippocampal commissures, and an ipsilateral lesion of the entorhinal cortex) or a crossed pathway (following a unilateral lesion of the fornix and a contralateral lesion of the entorhinal cortex, leaving the hippocampal commissures intact) was sufficient. When the lesions produced a complete bilateral disconnection of the fornix and entorhinal cortex, rats performed poorly. The results indicate that the hippocampal system provides a functional connection between the subcortical structures associated with the fornix and the neocortical structures associated with the entorhinal cortex, and that without this connection normal processing of working memory can not occur.