We reported recently that transgenic mice overexpressing human neurofilament heavy (NF-H) proteins develop a progressive neurological disorder with pathological features resembling those found in amyotrophic lateral sclerosis (ALS) (Côté et al 1993). A simple behavioral test, the grasping ability, has been used here for evaluating the motor dysfunction during aging of NF-H transgenic mice. Transgenic mice overexpressing NF-H proteins are normal at birth but they progressively fail to uphold their weight when tested for their grasping ability. The deficits in motor function during aging correlate with a progressive disruption of peripheral nerve function as evidenced by the atrophy and degeneration of distal axons.