Memory loss is a common feature of aging in the human but not all memories are equally lost. Usually the loss is more severe for the memory of recent events and information rather than the memory of remote events and information. Associated with memory losses of this type is a reduction in levels of cortical acetylcholine and a loss of cholinergic cells of the nucleus basalis magnocellularis. In the rat it is possible to assess two types of memory analogous to recent and remote memories in humans. Reference memory would refer to information concerning the task that is invariant from trial to trial. Working memory would refer to information that changes from trial to trial. In support of the hypothesis that decreases in cholinergic function lead to a differential impairment of working memory, rats trained in this task and given scopolamine showed a specific increase in working memory errors. In more recent studies rats have been trained in a task and then given unilateral neurotoxic lesions of the NBM. These results raise the intriguing possibility that degeneration of the NBM and associated memory impairment seen in aging and in Alzheimer's disease may be related to a change in the ratio of these or related endogenous tryptophan metabolites.