Mutations in the beta-amyloid precursor protein are strongly associated with some cases of familial Alzheimer's disease. The normal physiological role of beta-amyloid precursor protein in the brain was evaluated in a cross-sectional analysis of mice deficient in beta-amyloid precursor protein. Compared with wild-type control mice the beta-amyloid precursor protein-null mice developed age-dependent deficits in cognitive function and also had impairments in long-term potentiation. In addition, the brains of the beta-amyloid precursor protein-null mice had marked reactive gliosis in many areas, especially in the cortex and hippocampus. A subpopulation of mice (n = 15) died prematurely (between three and 18 months of age). Analysis of another six mice from the same population that were showing weight loss and hypolocomotor activity exhibited a marked reactive gliosis as detected by immunoreactivity for glial fibrillary acidic protein and a profound loss of immunoreactivities for the presynaptic terminal vesicle marker proteins synaptophysin and synapsin and the dendritic marker microtubule-associated protein-2 in many brain areas, but most predominantly in the cortex and hippocampus. These results suggest that normal beta-amyloid precursor protein may serve an essential role in the maintenance of synaptic function during ageing. A compromise of this function of the beta-amyloid precursor protein may contribute to the progression of the memory decline and the neurodegenerative changes seen in Alzheimer's disease.