The maximum force of voluntary muscle contraction was registered under isometric conditions in nine patients with recessive myotonia congenita. The recordings were made on the upper arm. Five patients with severe myotonia had a transient weakness after muscle rest. Electromyographic registrations with wire electrodes showed that the myotonic muscle fiber discharges disappeared during the transient weakness. Medication improving myotonic stiffness also improved the weakness. The cause of transient weakness seems to be similar to that of myotonic stiffness. It is known that an increasing depolarization of the myotonic muscle fiber membrane leads to the myotonic discharges and myotonic stiffness. In severe myotonia the progressing depolarization could cause a loss of excitability of the muscle fiber membrane and thereby a transient paresis of a more or less large number of muscle fibers.