Cytosolic Cu/Zn superoxide dismutase (SOD1) is a ubiquitous small cytosolic metalloenzyme that catalyzes the conversion of superoxide anion to hydrogen peroxide (H(2)O(2)). Mutations in the SOD1 gene cause a familial form of amyotrophic lateral sclerosis (fALS). The mechanism by which mutant SOD1s causes ALS is not understood. Transgenic mice expressing multiple copies of fALS-mutant SOD1s develop an ALS-like motoneuron disease resembling ALS. Here we report that transgenic mice expressing a high concentration of wild-type human SOD1 (hSOD1(WT)) develop an array of neurodegenerative changes consisting of (1) swelling and vacuolization of mitochondria, predominantly in axons in the spinal cord, brain stem, and subiculum; (2) axonal degeneration in a number of long fiber tracts, predominantly the spinocerebellar tracts; and (3) at 2 years of age, a moderate loss of spinal motoneurons. Parallel to the development of neurodegenerative changes, hSOD1(WT) mice also develop mild motor abnormalities. Interestingly, mitochondrial vacuolization was associated with accumulation of hSOD1 immunoreactivity, suggesting that the development of mitochondrial pathology is associated with disturbed SOD1 turnover. In this study we also crossed hSOD1(WT) mice with a line of fALS-mutant SOD1 mice (hSOD1(G93A)) to generate "double" transgenic mice that express high levels of both wild-type and G93A mutant hSOD1. The "double" transgenic mice show accelerated motoneuron death, earlier onset of paresis, and earlier death as compared with hSOD1(G93A) littermates. Thus in vivo expression of high levels of wild-type hSOD1 is not only harmful to neurons in itself, but also increases or facilitates the deleterious action of a fALS-mutant SOD1. Our data indicate that it is important for motoneurons to control the SOD1 concentration throughout their processes, and that events that lead to improper synthesis, transport, or breakdown of SOD1 causing its accumulation are potentially dangerous.