A method that improves contrast in gradient-recalled-echo magnetic resonance images is described. The increase in contrast is produced by applying low-power radio-frequency irradiation 5-10 kHz from the main water proton resonance frequency to excite macromolecular hydrogen-1 nuclei as part of a conventional gradient-recalled-echo sequence. The contrast so obtained is theoretically different from T1 or T2 contrast and has been termed magnetization transfer contrast. Cat head images were acquired in vivo with this method at 4.7 and 1.5 T. Compared with conventional gradient-recalled-echo images, the magnetization transfer images demonstrate increased contrast between many tissue pairs, such as between white matter and blood and between gray matter and cerebrospinal fluid. The dependence of the magnetization transfer effect on repetition time and preirradiation power were also studied.