Dehydration in cats may be caused by heat exposure, loss of fluids from vomiting or diarrhea, and/or due to an underlying serious medical problem. A dehydrated cat is a medical emergency. Oral rehydration via electrolytes is fruitless as cats are not very good drinkers. A dehydrated cat requires veterinary hospitalization and intravenous or subcutaneous fluid therapy in order to re-hydrate them.