The disturbances of water and electrolyte metabolism in the body can be subdivided into three large groups: 1) disturbances of free water which are related to the whole body water and are always coupled with changes in the osmolality of the internal environment and also of the interior of the cells (hypertension due to loss of water, hypotension in water intoxication); 2) isotonic changes of the volume of the extracellular fluid-oedema or extracellular dehydration. The latter are always coupled with a hypovolaemia and with a danger to the circulation which may end in shock. There is also a cumulative loss of potassium as a result of the dehydration reaction; 3) disturbances of the intracellular water metabolism which are associated with disturbances of the potassium metabolism. These may have grave consequences for the function of striated and smooth muscles, for the function of the nerves and various enzyme systems. Since the disorders of the extracellular water balance are generally best known, the attention is principally drawn in this paper to the hypertonic dehydration and the status of the potassium metabolism. The symptomatology of both conditions is discussed, and also their diagnosis with the simplest laboratory effort not associated with great loss of time (which is essential if valuable time is not to be lost before the laboratory results can be obtained). The theoretical considerations are supplemented with clinical examples and explanations of the treatment.