Histological and ultrastructural studies were performed on nerve and muscle biopsy specimens from two patients with the syndrome of continuous muscle fibre activity. The characteristics of muscle biopsies were as follows. By light microscopy, internal nuclei were present in many of the fibres. By electron microscopy many fibres contained filamentous bodies and subsarcolemmal aggregates of mitochondria embedded in the peripheral zone of cytoplasm, and occasional mitochondria with disorganized or branched cristae were larger than normal. Biopsies of sural nerves revealed a decreased number of myelinated fibres, clusters of small myelinated fibres, and evidence of active axonal degeneration such as disintegrated myelin segments and degenerated axon components, as well as loss of axonal contents. With the present biopsy findings, it is suggested that the pathological process of this syndrome affects peripheral nerves as well as muscles.