Amyotrophic lateral sclerosis (ALS) is a common form of motor neuron disease (MND) that involves both upper and lower nervous systems. In the SOD1G93A G1H transgenic mouse, a widely used animal model of human ALS, a significant pathology is linked to the degeneration of lower motor neurons in the lumbar spinal cord and brainstem. In the current study, the number of presynaptic boutons immunoreactive for synaptophysin was estimated on retrogradely labeled soma and proximal dendrites of α and γ motor neurons innervating the medial gastrocnemius muscle. No changes were detected on both soma and proximal dendrites at postnatal day 60 (P60) of α and γ motor neurons. By P90 and P120, however, α motor neuron soma had a reduction of 14 and 33% and a dendritic reduction of 19 and 36%, respectively. By P90 and P120, γ motor neuron soma had a reduction of 17 and 41% and a dendritic reduction of 19 and 35%, respectively. This study shows that levels of afferent innervation significantly decreased on surviving α and γ motor neurons that innervate the medial gastrocnemius muscle. This finding suggests that the loss of motor neurons and the decrease of synaptophysin in the remaining motor neurons could lead to functional motor deficits, which may contribute significantly to the progression of ALS/MND. © 2005 Wiley-Liss, Inc.