Membrane potentials, current‐voltage relationships, and component conductances were determined in resting excised external intercostal muscle fibers from five patients with paramyotonia congenita. At 37°C all investigated parameters were normal. At 27°C the resting potentials decreased to about −40 mV, and the fibers were inexcitable. At this stage the membrane currents were much larger than in normal fibers owing to increases in the membrane conductances for Na and Cl ions. The earlier finding that in the cold the Na permeability is abnormally large was confirmed. The Cl permeability was shown to be normal even in the cold. The decrease of the resting potential and the changes in the current‐voltage relationship at 27°C could be prevented by the use of the Na channel blocker tetrodotoxin (TTX) or by bathing the fibers in a Na‐free solution. Our previous conclusion that the Cl conductance at 27°C was also increased when TTX was present was not confirmed. Exposure of a muscle bundle to 7 mmol/l potassium did not lead to excessive depolarization and paralysis.