Amyloid precursor protein (APP) gives rise by proteolytic processing to the amyloid beta peptide (A beta) found abundantly in cerebral senile plaques of individuals with Alzheimer's disease. APP is highly expressed in the brain. To assess the source of cerebral A beta, the metabolism of APP was investigated in the major cell types of the newborn rat cerebral cortex by pulse/chase labeling and immunoprecipitation of the APP and APP metabolic fragments. We describe a novel C-terminally truncated APP isoform that appears to be made only in neurons. The synthesis, degradation, and metabolism of APP were quantified by phosphorimaging in neurons, astrocytes, and microglia. The results show that although little APP is metabolized through the amyloidogenic pathways in each of the three cultures, neurons appear to generate more A beta than astrocytes or microglia.