Intracellular recordings were made on the fast posterior latissimus dorsi muscles of normal and dystrophic chickens. In the dystrophic chickens, the rate of rise of the action potential was decreased. With repetitive indirect stimulation, the action potentials decreased in size and disappeared; only an end-plate potential remained. Membrane resistance, membrane capacitance, and duration of miniature end-plate potentials were increased. A decrease in sodium permeability may be in part responsible for the observed alterations in the electrical properties of the nerve terminal and postsynaptic muscle membrane.