A major factor contributing to proton (1H) spin-lattice relaxation in biological tissues is believed to be magnetization transfer between 1H in free bulk water and 1H restricted motion associated with macromolecules. We have shown recently that saturation transfer is an effective approach for studying this magnetization transfer process. Herein the determination of magnetization transfer rates in biological tissues is further analyzed by considering the time and power dependencies of saturation transfer. Following these analyses, quantitative magnetization transfer rate constant image maps were collected from the kidney in vivo. These rate constant images may prove useful in quantitative tissue characterization and in the determination of tissue-specific 1H relaxation mechanisms.