The Bielschowsky method is used to stain neurofibrils. Sections are sensitized with silver nitrate followed by secondary impregnation with an ammoniacal silver solution. Silver is deposited on the neurofibrillary tangles (NFT) and axonal or dendritic processes of senile plaques. Silver is reduced to visible, metallic silver by formalin. As a result, neurofibrils, NFT and neuronal processes appear black. Unreduced silver is removed with sodium thiosulfate.