Fluid and electrolyte imbalances are the most frequent complications following pituitary surgery. Among the several patterns of occurrence, hyponatremia can occur in an isolated fashion or as part of a bi- or triphasic pattern. The frequency of hyponatremia after trans-sphenoidal surgery is between 2% and 25%, according to the literature. However, these numbers are probably underestimating the real prevalence, since mild hyponatremia does not lead to symptoms and measurement of sodium level. No association has been described between entity of the pituitary tumor or tumor size and hyponatremia. Therefore no predictors exist to determine patients with a higher risk for electrolyte imbalances after surgery. However, since delayed hyponatremia occurs mainly around the 8-10th day after surgery, routine measurement of sodium should be recommended on the day of hospital dismission. In case of a symptomatic hyponatremia, insufficiency of the corticotrophe pituitary function as the leading differential diagnosis needs to be ruled out. If the patient is euvoleme, pretest probability of syndrome of inadequate antidiuretic hormone production (SIADH) is very high and therapy may be started according to this. In case of SIADH, therapeutic options include fluid restriction or vaptane therapy. Only in severe cases infusion of hypertonic saline is appropriate. Usually SIADH following pituitary surgery is a self-limiting condition and will cease within 2-5 days.