Experimental paradigms adopted from animal models were used to compare the neuropsychological mechanisms underlying the dementias of Alzheimer's and Parkinson's diseases. Two tasks were selected because characteristic profiles of impairment in nonhuman primates are seen following selective lesions of frontal cortex, temporal cortex, and fornix. The tasks consisted of a spatial and a visual learning problem, each with two components: (1) original learning and (2) reversal of the original learning. The Alzheimer's patients were significantly impaired on original learning and reversal learning in the visual modality compared with demented Parkinson's patients, even though both groups were equated for severity of dementia. On the spatial tasks, both the Alzheimer's and the demented Parkinson's patients were impaired on reversal learning but not on original learning. The profile of deficits on the visual tasks may serve to differentiate Alzheimer's from Parkinson's dementia and may reflect selective orbitofrontal system lesions in the former.