Electromyographic responses to needle-electrode insertion and repetitive indirect stimulation were recorded from gastrocnemius, soleus and cranial tibialis muscles of normal and dystrophic (mdx) mice at 20-154 days. Recordings from myotonic (ADRmto) mice served as controls for “true” myotonia. Complex repetitive discharges were observed in muscles of mdx mice but no complex repetitive discharges or other abnormalities were observed in muscles of normal control mice. Complex repetitive discharges in mdx muscles did not resemble the prolonged, abnormally repetitive discharges observed in myotonic mouse muscles. Neuromuscular blockade with tubocurarine and nerve transection had no effect on observed electromyographic abnormalities in the mdx mice. Nerve conduction and repetitive stimulation studies also revealed no abnormalities in mdx mice. These findings indicate that the mdx condition is a nonmyotonic, primary myopathy. No signs of myotonia or impaired neuromuscular transmission were found in mdx mice.