Two markers of neuronal plasticity were used to compare the response of the human central nervous system to neuronal loss resulting from Alzheimer's disease with the response of rats to a similar neuronal loss induced by lesions. In rats that had received lesions of the entorhinal cortex, axon sprouting of commissural and associational fibers into the denervated molecular layer of the dentate gyrus was paralleled by a spread in the distribution of tritiated kainic acid-binding sites. A similar expansion of kainic acid receptor distribution was observed in hippocampal samples obtained postmortem from patients with Alzheimer's disease. An enhancement of acetylcholinesterase activity in the dentate gyrus molecular layer, indicative of septal afferent sprouting, was also observed in those patients with a minimal loss of cholinergic neurons. These results are evidence that the central nervous system is capable of a plastic response in Alzheimer's disease. Adaptive growth responses occur along with the degenerative events.