The murine mutant with motor end-plate disease (med) exhibits a progressive weakness which is due to a functional denervation of skeletal muscle. It is inherited as an autosomal recessive trait and has two alleles. One of these is the jolting (medjo) mouse which also suffers from a disturbance of locomotion. Although the jolting animal was originally thought to suffer from a disorder of neuromuscular transmission, the organization of the peripheral nervous system is indistinguishable from normal and neuromuscular transmission is unimpaired. It is now clear that abnormalities in the cerebellum are responsible for the gait problems of the jolting mouse, and a preliminary morphological and neurophysiological examination of the mouse with motor end-plate disease has shown that it also has cerebellar abnormalities. Axonal spheroids may be seen on cerebellar Purkinje cell axons in the med mouse, and the character of extra-cellular recordings obtained from the Purkinje cells is abnormal.