Administration of fluid and vasopressin injection to rats induced within 24 to 48 hours a condition which can be considered as an experimental model of the syndrome of inappropriate secretion of antidiuretic hormone (ADH). Concurrently with hyponatremia and hypoosmolarity in serum, the changes in muscle consisted of a fall in sodium content and a decrease in percentage dry weight equivalent to 12% swelling. There was no loss of potassium from muscle. In contrast, in brain the decrease in sodium content was smaller, the tissue percentage dry weight decreased only slightly, indicating minimal swelling, and there was a significant net loss of potassium. It is suggested that neurological dysfunction associated with the syndrome of inappropriate secretion of ADH is unlikely to be due to cerebral edema but may be related to the decreased potassium content of brain tissue.