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Timedomain nuclear magnetic resonance (TD-NMR)-based measurement of body composition of rodents is an effective method to quickly and repeatedly measure proportions of fat, lean, and fluid without anesthesia. TD-NMR provides a measure of free water in a living animal, termed % fluid, and is a measure of unbound water in the vascular and extracellular spaces. We hypothesized that injecting a bolus of fluid into the peritoneal cavity would lead to an abrupt increase in % fluid and the rate of clearance monitored with TD-NMR would provide a noninvasive assessment of the free water homeostasis in an awake rat. Several strains of laboratory rats were injected intraperitoneally with 10 ml/kg isotonic or hypertonic saline and % fluid was monitored repeatedly with a Bruker "Minispec" TD-NMR body composition system. Following isotonic saline, % fluid increased immediately by 0.5% followed by a recovery over similar to 6 h. Injecting hypertonic (3 times normal saline) resulted in a significantly greater rise in % fluid and longer recovery. Intraperitoneal and subcutaneous fluid injection led to similar rates of clearance. The Wistar-Kyoto rat strain displayed significantly slower recovery to fluid loads compared with Long-Evans and Sprague-Dawley strains. Rats exercised chronically showed significant increases in % fluid, but the rate of clearance of fluid was similar to that of sedentary animals. We conclude that this technique could be used to study vascular and extracellular volume homeostasis noninvasively in rats.