The present experiment was designed to determine the extent to which two different mechanisms were responsible for the recovery of function produced by grafts of fetal basal forebrain tissue into the hippocampus of rats given fimbria-fornix lesions. The place fields of single units in the hippocampus were recorded as rats moved about a radial arm maze. Two measures of the discriminative stimuli identifying the place fields were determined: place stability, maze stability. In control (CON) rats, place stability was high and maze stability was low, indicating that the relevant discriminative stimuli were extra-maze cues. In rats with fimbria-fornix (FF) lesions, place stability was low and maze stability was high, indicating that the relevant discriminative stimuli came from the maze itself. In rats given grafts of fetal basal forebrain tissue (GRAFT), place stability was increased and maze stability was reduced as compared to FF rats. These results indicate that the grafts produced recovery of function through restoration of normal hippocampal unit activity, rather than by reorganization of new neural networks.