Contrast-enhancing pharmaceuticals and manipulation of the physical properties of tissue may extend the diagnostic usefulness of nuclear magnetic resonance (NMR) imaging. Potential applications of contrast enhancers include differentiation of isomagnetic tissues, direct evaluation of tissue function, evaluation of metabolic pathways, and measurement of tissue perfusion. Numerous means of enhancing NMR contrast are available, but many, such as manipulation of tissue viscosity or temperature, are not clinically feasible. Changing the chemical composition of tissue, and particularly the use of paramagnetic pharmaceuticals, appears to be more advantageous. Nitroxide stable free radicals are a group of synthetic, strongly paramagnetic compounds that have several characteristics that make them promising contrast enhancers.