Hereditary canine spinal muscular atrophy (HCSMA) is an inherited disorder of motor neurons that shares features with human motor neuron disease. In previous work, we have demonstrated that motor units become unable to sustain force output during repetitive activation. This failure appears before degenerative changes can be detected at neuromuscular junctions. A variety of evidence indicates that motor unit failure may be caused by neurotransmission failure, including the ability of 4-aminopyridine to improve force generation in failing motor units. To directly examine neuromuscular transmission in HCSMA, we used two-electrode voltage clamp of endplate currents to measure endplate currents in the medial gastrocnemius muscle of homozygotes. Miniature endplate currents were normal in amplitude, suggesting that postsynaptic receptor acetylcholine receptor density is normal. However, endplate currents were significantly reduced in affected muscle secondary to a decrease in quantal content. Motor units in HCSMA exhibit an increase in amplitude after tetanic stimulation. We found that this posttetanic facilitation was caused by an increase in quantal content after stimulation of failing neuromuscular junctions. We conclude that in this model of motor neuron disease weakness results from failure of neuromuscular transmission.