The ultrastructural features of a recently described inherited lower motor neuron disease were studied in 5 affected pigs. Clinical signs comprised progressive ataxia and paresis of variable severity. Affected pigs, 6, 7, 15, 15, and 19 weeks of age, and 2 unrelated healthy pigs, 9 and 15 weeks of age, were anesthetized and their tissues were fixed by whole body perfusion with mixed aldehydes. From 1 or more affected pigs, samples of cervical and lumbar spinal ventral horn, lateral and ventral spinal columns, dorsal and ventral lumbar spinal nerve roots, 2 peripheral nerves (Nn. phrenicus and fibularis communis), and 2 skeletal muscles (Mm. diaphragma and tibialis cranialis) were examined ultrastructurally. There was widespread degeneration of myelinated axons in peripheral nerves and in lateral and ventral columns of lumbar and cervical segments of spinal cord. Axonal degeneration was present in ventral spinal nerve roots and was absent in dorsal spinal nerve roots sampled at the same lumbar levels. Unmyelinated axons in peripheral nerves and spinal nerve roots were unaffected. In 4 of 5 affected pigs, there were atrophic alpha motor neurons in cervical spinal cord that contained dense, round osmiophilic perikaryal inclusions up to 4 microns in diameter and round swollen mitochondria. Axonal regeneration was present in N. phrenicus of the 19-week-old affected pig that had clinical signs of longest duration (10 weeks). There was no morphologic evidence of axonal degeneration or spinal neuronal atrophy in either control pig. The ultrastructural features of this motor neuron disease distinguish it from other reported progressive spinal neuropathies of pigs.