A nine-year-old female beagle with acromegaly and extreme polyuria and polydipsia during dioestrus is described. It was demonstrated that polyuria was related to an inadequate rise of arginine-vasopressin (AVP) levels after water deprivation and stimulation with hypertonic saline. Administration of AVP did not lead to a significant increase in urine osmolality or reduction of urine volume. Clinical signs, except for bony changes, completely disappeared following ovariohysterectomy.