Quantitative magnetic resonance (QMR) is a non invasive method of determining body composition in awake animals. The principles of the method are similar to magnetic resonance imaging (MRI), except that the output is numerical rather than an image. Non sedated rodents are restrained in a Plexiglas tube that is placed within the bore of a low field magnet. Data collection takes two minutes, allowing a high throughput and repeated measurement for longitudinal study designs. During the data collection, hydrogen atoms in fat and lean tissue respond differently to radiofrequency energy, allowing the determination of the subject's fat, lean and total body water mass.