An imaging instrument for measuring the 2D distribution of both free radicals and protons in a single sample was developed. A common resonator was designed for this instrument that can resonate at an L-band electron spin resonance (ESR) frequency or a HF (27.7 MHz) proton nuclear magnetic resonance (NMR) frequency. This resonator enabled construction of a simple detection block for the instrument. Because the detection processes of ESR and NMR are similar, many components, including the main magnet, magnetic-field modulation coils, field gradient coils, electrical circuits, data acquisition system, computer for reconstructing images, and most of the software were shared by these two systems. By using this instrument, both ESR and NMR images of the head of a live rat were successfully produced. The free radical distribution could be positioned by superimposing the ESR image on the NMR image. © 1997 American Institute of Physics.