The resting membrane potentials of X-linked muscular dystrophy (mdx) mice are about 10 mV lower than those of control (C57BL/10Sc) mice. Also repetitive action potentials which resemble insertion myotonia are frequently recorded intracellularly from the mdx mice of various ages. When the mdx mice are treated with bestatin intraperitoneally for 7 days, the RMP's improvement almost attained its normal levels. Repetitive action potential bursts (electrical myotonia) recorded at the time of microelectrode insertion had decreased or abolished after bestatin therapy. Another set of in vitro experiments to see direct action of bestatin to the hemidiaphragm preparations of the mdx mice reveal improvement in RMP's and a reduction in electrical myotonia within a 30-minute exposure to Tyrode's solution with bestatin. Bestatin has a favorable direct effect on the muscle fiber membrane to stabilize RMP's and suppress muscle membrane irritability.