Focal lesions in the plasma membrane overlying wedge-shaped defects in muscle fibers ("delta lesions") are an early pathological change in Duchenne muscular dystrophy (DMD). Abnormalities in the plasma membrane have been suggested as a cause of these lesions and of the degeneration of muscle fibers in DMD. We investigated the role of plasma membrane defects in the production of delta lesions by examining the effects of a series of membrane-active agents--lysolecithin, deoxycholate, Triton X-100, and melittin--on the muscles of rats in vivo. Within minutes after treatment with these agents, the muscle fibers developed typical delta lesions. Identical morphological changes were produced by the calcium ionophore A23187, suggesting that calcium entry may play an important role in this process. We conclude that damage to the plasma membrane or calcium entry can reproduce characteristic features of the muscle pathology seen in DMD. This model should prove useful in elucidating the mechanisms of muscle fiber damage and degeneration in DMD.