Mice with the inherited disorder "motor end-plate disease" suffered from a progressive neuromuscular weakness and muscular wasting. The weakness resulted from a failure of evoked transmitter release from the motor nerve terminals. The failure in transmission was all-or-nothing in nature. The numbers of muscle fibres in skeletal muscle and myelinated axons in several major nerve trunks were no different from normal. The loss in muscle bulk was caused by the neuromuscular defect and not from a loss of motor units or muscle fibres. The inherited murine disorder "jolting" was allelic with "motor end-plate disease". Affected "jolting" mice suffered no detectable morphological abnormality in skeletal muscle or peripheral nerve. The physiological properties of skeletal muscle and the characteristics of neuromuscular transmission were indistinguishable from normal.