Single-channel activity was recorded from cell-attached patches on skeletal muscle cells isolated from wild-type mice and from mice carrying the dy or mdx mutations. Spontaneous openings of the nicotinic acetylcholine receptor channel (nAChR) were detected in virtually all recordings from either dy/dy or dy/+ myotubes, but only infrequently from wild-type or mdx myotubes. Spontaneous openings were also present in most recordings from undifferentiated myoblasts from all of the mouse strains studied. The biophysical properties of the spontaneous activity were similar to those of the embryonic form of the nAChR in the presence of acetylcholine (ACh). Examination of the single-channel currents evoked by low concentrations of ACh showed a reduced sensitivity to the agonist in the dystrophic dy and mdx myotubes, but not in wild-type myotubes. The results suggest that alterations in nAChR function are associated with the pathogenesis of muscular dystrophy in the dy mouse.