Our purpose was to investigate a method of prolonged desmopressin (DDAVP) infusion in a free roaming rat to better understand the SIADH (syndrome of inappropriate antidiuretic hormone secretion) syndrome in man. DDAVP was infused for 2 weeks from implanted self-powered osmotic minipumps. At the end of that time, plasma DDAVP and urine osmolality were both significantly elevated in experimental as compared with control animals. However, hyponatremia and hypoosmolality, which are characteristic in the SIADH, did not develop. Our observations suggest that inappropriate high antidiuretic hormone levels do not necessarily lead to the SIADH either by urine sodium loss or by water retention if animals decrease water intake.