Neurites in senile plaques in the cerebral cortex from an autopsied case of Alzheimer's disease were examined by electron microscope. Swollen axons containing vastly increased numbers of neurofilaments (NF) together with paired helical filaments (PHF) were common. In some of these axons the continuity of NF with PHF was frequently discernible. In one, there were numerous figures of continuity in a bundle, indicating transition from NF to PHF. Moreover, some of these figures showed that two NF seemed to join to form one PHF. No straight 15 nm filament was noted anywhere in the sectioned plane of this axon. Instead, NF appeared to have contacts with many coated vesicles here and in the other swollen and/or degenerated axons that showed the coexistence of PHF with NF. These observations suggest not only that PHF originate from NF but that individual NF can be converted simultaneously and directly into PHF, possibly by a chemical modification without their first forming straight filaments.