To investigate the toxicity of β-amyloid protein, a component of the senile plaques in Alzheimer''s disease, it was infused into the cerebral ventricle of rats for 14 days by a mini-osmotic pump. Performances in the water maze and passive avoidance tasks in β-amyloid protein-treated rats were impaired. Choline acetyltransferase activity significantly decreased in the hippocampus both immediately and 2 weeks after the cessation of the infusion. However, the learning impairment was recoverable 2 weeks after cessation of the infusion. Both immediately and 2 weeks after the cessation of the infusion, glial fibrillary acidic protein immunoreactivity increased. Furthermore, β-amyloid protein altered the staining in the nuclei of hippocampal cells for only 2 weeks after the cessation. These results suggest that β-amyloid protein produces some damage in the central nervous system in vivo.