Intrahippocampal grafts of fetal basal forebrain tissue have been shown to restore several aspects of neural function, including some degree of behavioral recovery in spatial memory tasks, in rats with fimbria-fornix lesions. Place fields, the behavioral correlates of complex-spike unit activity recorded in the hippocampus of rats, are altered by fimbria-fornix lesions, and provide an important measure of the functioning circuitry of the hippocampus after grafts. To investigate the effects of grafts on hippocampal circuitry, complex-spike units were recorded while the rats traversed a radial maze. Quantitative analyses of spatial activity showed that units in normal rats had spatially clustered, reliable place fields that were stable despite alterations of the maze. In contrast, units in rats with fimbria-fornix lesions had more dispersed, less reliable place fields that were disrupted when the maze was covered or rotated. Compared to rats with fimbria-fornix lesions, rats with grafts and units with more tightly clustered, more reliable, and more stable place fields when the maze was altered. The results suggest that: (1) fimbria-fornix lesions disrupt some aspects of complex-spike place field activity; (2) the functioning of hippocampal circuitry is influenced by fetal basal forebrain grafts; and (3) the grafts may ameliorate the effects of lesions on spatial behaviors by influencing critical aspects of place field activity in the hippocampus.