A case of transient vasopressin-resistant diabetes insipidus is reported which developed during the seventh gestational month. Polyuria reached 4-6 L daily and urine osmolality remained dilute despite 21 hours of water deprivation followed by 5 U intramuscularly of aqueous pitressin, as well as four days of treatment with intranasal DDAVP (0.1-0.5 mL every 12 hours). Urinary excretion of prostaglandin E2, 1384 ng/24 hours, was fourfold that in nongravid subjects and a plasma arginine vasopressin level of 12 pg/mL was recorded. Indomethacin had no effect on urine osmolality but decreased urine volume markedly. Hydrochlorothiazide, also, decreased urine volumes, and this drug was used to manage the patient until delivery. The syndrome remitted in the puerperium, the patient concentrating her urine to 938 mOsm/kg when tested several months postpartum.