The myotonias are a group of genetically distinct hereditary diseases characterized by specific muscle malfunctions. They are interesting to the physiologist because much has been learned about muscle function from the study of the pathomechanisms underlying myotonia. This review classifies all diseases associated with myotonia, quantifies the myotonic signs as far as they have been worked out, and reviews the membrane changes detected in the various diseases. Finally, we discuss to what extent the clinical observations can be accounted for by the described membrane changes. A few short reviews on myotonia appeared recently (l&34, 51). In addition, several comprehensive textbooks on muscle disorders contain chapters on myotonia, but they are written mainly for the clinician (223, 282, 3%).