To perform a quantitative investigation on synaptic ultrastructural features in the human hippocampus in normal old and senile demented patients we stained our tissue samples by means of the ethanol-phosphotungstic acid (E-PTA) preferential technique. In addition to the synaptic contact zones, we found that structures similar to neurofibrillary tangles (NFT) and senile plaques (SP) were remarkably positive to our staining procedure, while the background was faintly electron lucent. On the basis of the E-PTA staining properties and specificity, we support that the reactive sites of these positive structures are represented by basic amino acids. The recently demonstrated presence of 4 basic amino acids (1 arginine, 3 histidine) in the cleaved fragment of the amyloid precursor protein (APP) suggest us to hypothesise that this APP portion may represent the common constitutive element of the many morphologically different alterations found in the brains of senile demented patients.