Impaired axonal transport of the fast or slow component has been reported in patients with sporadic amyotrophic lateral sclerosis (ALS), animal models for ALS, and familial ALS-linked mutant Cu/Zn superoxide dismutase (SOD1) transgenic mice. However, little is known about the impairment of axonal transport in mutant SOD1 transgenic mice. This is the first electron microscopic investigation of the axon hillock (AH) and the initial segment (IS) of anterior horn cells in the spinal cord of transgenic mice expressing the G93A mutant human SOD1, and it was launched with a view toward examining whether the axonal transport is impaired in this region. Six transgenic mice were killed at ages ranging from the presymptomatic to symptomatic stages. Six age-matched non-transgenic wild-type mice served as controls. In the non-transgenic mice, 91 AH and IS were observed, but those with increased neurofilaments or mitochondria were rarely found. In the transgenic mice, 95 AH and IS directly emanating from normal-looking large anterior horn cells were seen. AH and IS with increased neurofilaments or, to a lesser extent, increased mitochondria, and round-shaped mitochondria in particular, were more frequently observed, even at the early presymptomatic stage, than in the controls, and the frequency increased with time through the presymptomatic stages. On the other hand, the somata of large motor neurons directly connected with the axons did not exhibit any abnormal accumulation of neurofilaments or mitochondria. These findings suggest that both the slow axonal transport of neurofilaments and the fast axonal transport of mitochondria are impaired in AH and IS before the onset of disease in this animal model.