Studies of the neurochemical pathology of Alzheimer's disease and Parkinson's disease reveal a severe and specific loss of central nicotinic cholinergic receptors. We have investigated the functional significance of this finding for cognitive functioning by studying the effects of the centrally active nicotinic antagonist mecamylamine. Single oral doses of mecamylamine were administered to 12 healthy young males and 15 healthy elderly subjects in doses of 5, 10, and 20 mg in a placebo-controlled, double-blind study. In both groups, the 20-mg dose caused a significant increase in errors in the learning condition of the Repeated Acquisition Task, producing a slower acquisition curve. There was no effect of drug on the performance component (retrieval of previously learned information). However, elderly subjects showed en hanced sensitivity to mecamylamine,