Plasma osmolality normally decreases in early pregnancy, reaching a minimum at approximately 10 weeks and remaining depressed until term. This is associated with a mean decrease of 4 mEq/L in the plasma sodium level, and with an altered threshold for arginine vasopressin (AVP) release and for thirst. We describe a patient who developed more severe hyponatremia (120 mEq/L), which accompanied the development of hypertension and edema at 37 weeks in her fourth pregnancy. Hyponatremia and hypo-osmolality were associated with marked elevation of the plasma AVP level. The hyponatremia and elevated AVP level resolved after the delivery of the infant. To our knowledge, this is the first reported example of transient inappropriate antidiuretic hormone secretion (SIADH) associated with pregnancy.