Dehydration, in childhood as in adulthood, may origin from an inadequate water ingestion or an excessive water elimination. Causes may be found in fever, vomiting, scalds, pulmonary hyperventilation, diabetes. Water loss during acute diarrhea in children can be even 6-7 times higher in comparison with an healthy child. Together with water, electrolytes are lost. We differentiate dehydration in isonatremic d. (70% of cases), hyponatremic d. (10%) and hypernatremic d. (20%) basing on Sodium loss. Important dehydration causes severe clinical symptoms as shock, renal and cardiocirculatory failure, convulsion, coma. Symptoms at the central nervous system level derivate both from hyperosmolarity in brain cells and from thrombosis or hemorrhages in subdural sites. Dehydration, following acute diarrhea, is slight when weight loss is lower than 5%. The child health conditions still remain good. Dehydration become moderate if weight loss reaches 5% and the child starts suffering. When the weight loss reaches 10%, dehydration is now severe and circulatory deficiency becomes evident. When it is higher than 10%, prognosis is very severe and shock and coma may be observed. In the present work, we illustrate the different ways of rehydration after acute diarrhea. Initially, oral rehydration must be established with one of the oral solutions, differing each other for amount of electrolytes and glucose. Recently, a new solution, "supersolution", has been presented differing from the other ones for electrolytes concentration and for the presence of rice starch instead of glucose. In most cases of diarrhea, oral rehydration appears adequate but sometimes an intravenous rehydration becomes necessary, e.g. in case of vomiting, CNS depression and in any case of severe gastroenteric symptomatology.(ABSTRACT TRUNCATED AT 250 WORDS)