Transgenic mice carrying the human CuZn-superoxide dismutase gene were used to investigate whether CuZn-superoxide dismutase gene dosage is involved in the signs of neuromuscular junction deterioration associated with Down's syndrome. Three parameters of neuromuscular junction morphology were studied in hindlimb muscles of CuZn-superoxide dismutase-transgenic mice and their non-transgenic littermates: nerve terminal length, number of nerve terminal branching points and incidence of sprouting that results in synapse formation. These parameters increased with advanced age and the increase occurred earlier in CuZn-superoxide dismutase-transgenic mice. Therefore, the data is in line with the possibility that CuZn-superoxide dismutase-transgenic mice are undergoing premature ageing with respect to neuromuscular junction morphology, most probably owing to a gene dosage effect of CuZn-superoxide dismutase.