The goal of maintenance fluid therapy in small animals is to replace normal ongoing losses of water and salts when oral intake is withheld. Hospitalized dogs and cats may have multiple stimuli for antidiuretic hormone release that disrupt normal osmoregulation and predispose to water retention. Severe illness promotes retention of both sodium and water as edema. Commercially available fluids have electrolyte concentrations that are very different from dietary maintenance requirements, and potential consequences include development of hypoosmolality, edema, or both when excesses of water or sodium are administered. Suggestions for tailoring fluid administration toward specific goals are provided.