THE PURPOSE OF TMS PAPER is to draw attention to a histological phenomenon which may be seen in certain cases of motor neuron disease, namely focal enlargement of axons of cells in the anterior horns of the spinal cord and in the somatic motor nuclei of the brainstem. Correlation between the clinical course of the disease and the presence of these axonal changes seems possible. The mechanism of their production is unknown, although some relevant data are available. The size and shape of the axonal enlargements may be such that they can be mistaken for chromatolytic neurons. This is perhaps the reason why they are seldom mentioned in pathological descriptions of the disease, although in 1957 Wohlfartl stated that argyrophilic axonal bodies were an invariable, though nonspecific, finding in motor neuron disease. He saw them in the ventral horns, the brainstem, and, to a lesser extent, the cerebral cortex. They were from 2 to 30 p in diameter. He felt that most of them represented terminal knobs of axons but that a few were separated from their parent axon. He envisaged 3 possibilities for their genesis:[l] that they were formed from collateral outgrowth from damaged axons of ventral horn cells,[2] that they were formed from axons having previous synaptic contact with now defunct ventral horn cells, or [3] that they were formed from dying back of terminal axons of corticospinal fibers entering the anterior horns. He noted that axonal enlargements were most numerous at the ventral side of the ventral horns but that they were seen fairly often close to neuronal perikarya.