The case of a 71 years old male patient with paramyotonia congenita is described. There was no occurence of spontaneous paresis at room temperature. Lowering the temperature, a loss of muscle power became perceivable in the mechanogram while, at the beginning there was only a slight depression in amplitude of the evoked action potential. After termination of the cooling paresis became even more pronounced. Electric activity too faded away almost entirely. Repetitive stimulation showed a marked fatigability of the muscle. — The findings seem to indicate a direct influence of low temperature on the contractible substance of the muscle.