Transgenic mice expressing mutant (P301L) human tau develop neurofibrillary tangles, amyotrophy and progressive motor disturbance. We present ultrastructural features of neuronal degeneration in this model that suggests involvement of both neurofibrillary and autophagic processes in neurodegeneration. Neurons undergoing neurofibrillary degeneration contain tau-immunoreactive, 15-20 nm-wide straight or wavy filaments with no periodic twists. Tau filaments were found in two types of affected neurons. One type resembled neurons with neurofibrillary tangles (NFT) that were filled with numerous filaments that displaced sparse cytoplasmic organelles to the periphery. Microtubules were almost completely absent. The nucleus remained centrally located, but showed lobulations due to deep infoldings. The other type resembled ballooned neurons seen in some human tauopathies. The nucleus was peripherally placed, but normal appearing. The cytoplasmic organelles were dispersed throughout the swollen perikarya, the Golgi complex was fragmented and duplicated, while mitochondria and other organelles appeared normal. Tau filaments similar to those in NFT were sparse and not tightly packed. Microtubules were also sparse. Many autophagic vacuoles were present in these cells. Heterogeneous appearing axonal swellings resembling spheroids in human tauopathies were present in gray and white matter. Unlike normal appearing axons, axonal spheroids were filled with tau-immunoreactive filaments and autophagic vacuoles, in addition to normal appearing neurofilaments and microtubules. These P301L transgenic mice exhibit many features common to human tauopathies, making them a valuable model to study the pathogenesis of these uncommon disorders.