The role of the basal forebrain cholinergic system in hippocampal spatial representation was explored by examining the effects of immunotoxic lesions of the septo-hippocampal cholinergic neurons on the firing patterns of hippocampal place cells as rats explored familiar and novel environments. In a highly familiar environment, the basic qualities and stability of place fields were unaffected by the lesion. When first exposed to a set of novel environmental cues without otherwise disorienting the animals, place cells in both normal and lesioned animals responded with similar alterations in their firing patterns. Upon subsequent repetitive exposures to the new environment, place cells of normal rats developed a spatial representation distinct from that of the familiar environment. By contrast, place cells of lesioned animals reconverged in the direction of the representation associated with the familiar environment. These results suggest that cholinergic input may determine whether new visual information or a stored representation of the current environment will be actively processed in the hippocampal network.