The electrical gradient across the mitochondrial inner membrane (Ψ m ) is established by electron transport chain (ETC) activity and permits mitochondrial Ca 2+ sequestration. Using rhodamine-123, we determined how repetitive nerve stimulation (100 Hz) affects Ψ m in motor terminals innervating mouse levator auris muscles. Stimulation-induced Ψ m depolarizations in wild-type (WT) terminals were small (<5 mV at 30 °C) and reversible. These depolarizations depended on Ca 2+ influx into motor terminals, as they were inhibited when P/Q-type Ca 2+ channels were blocked with ω-agatoxin. Stimulation-induced Ψ m depolarization and elevation of cytosolic [Ca 2+ ] both increased when complex I of the ETC was partially inhibited by low concentrations of rotenone (25–50 nmol/l). This finding is consistent with the hypothesis that acceleration of ETC proton extrusion normally limits the magnitude of Ψ m depolarization during mitochondrial Ca 2+ uptake, thereby permitting continued Ca 2+ uptake. Compared with WT, stimulation-induced increases in rhodamine-123 fluorescence were ≈5 times larger in motor terminals from presymptomatic mice expressing mutations of human superoxide dismutase I (SOD1) that cause familial amyotrophic lateral sclerosis (SOD1-G85R, which lacks dismutase activity; SOD1-G93A, which retains dismutase activity). Ψ m depolarizations were not significantly altered by expression of WT human SOD1 or knockout of SOD1 or by inhibiting opening of the mitochondrial permeability transition pore with cyclosporin A. We suggest that an early functional consequence of the association of SOD1-G85R or SOD1-G93A with motoneuronal mitochondria is reduced capacity of the ETC to limit Ca 2+ -induced Ψ m depolarization, and that this impairment contributes to disease progression in mutant SOD1 motor terminals.