After developing a stable hyponatremia (serum sodium 125-126mEq/liter) a patient was studied during a standard oral water load and during isotonic and hypertonic saline infusions. The patient excreted the water load in four hours, excreted dilute and then concentrated urine in response to isotonic saline, and concentrated her urine in response to hypertonic saline. Normal responses to these maneuvers while the patient was hyponatremic suggested reset of her osmoreceptors at a decreased serum sodium level. Data during the development of the hyponatremia demonstrated negative sodium balance, but neither negative water balance nor a decrease in body weight was seen, suggesting, as did her response to water loading, that the patient was normovolemic. It is proposed that reset does occur and can be demonstrated if hyponatremia develops while intravascular volume is maintained at normal levels.