Morphological abnormalities of the neuromuscular junction in two murine models with primary myopathy were studied by combined cholinesterase-silver impregnation techniques and electron microscopy. In both situations the results were similar showing that the neuromuscular junction remained unaffected even when inervating necrotic muscle fibres. In regenerated muscle fibres, however, there was marked simplification of the post-synaptic membrane with reduction in number and depth of folds up to 50% of normal values confirmed by morphometric analysis. Since after regeneration succeded no detectable clinical or physiological alterations were observed in these experiments it seems reasonable to assume that the prominent branching of post-synaptic folds in normal skeletal muscles might represent an increased anatomical safety mechanism in chemical transmission.