In a patient with hypokalemic periodic paralysis (HPP), recovery from a paralytic attack, coinciding with the restoration of plasma potassium, was associated with a rise in serum myoglobin (Mb) and creatine kinase (CK). The increase of muscle proteins in serum was preceded by an increase in serum free fatty acids (FFA). An increased permeability of the sarcolemma, caused by an accumulation of FFA within the muscle cell, might underly the reflux of potassium into the circulation. This might represent a mechanism by which the paralytic attacks in HPP terminate.