The feasibility of a fiber-optic plate (FOP) microscope system employing a bundle of optical fibers and videomicroscopy for in vivo experiments was investigated. The FOP used here consisted of optical fibers 3 microns in diameter. By inserting the FOP into an animal, optical signals from the deep-lying tissue invisible from the surface could be obtained as two-dimensional images. Using this system, hippocampal cells stained with a fluorescent dye in an anesthetized rat were visualized. Elevation of intracellular free calcium concentration ([Ca2+]i) in the hippocampus of the rat during anoxic exposure was also detected with a fluorescent indicator dye. These results showed that the FOP microscope system was sufficiently applicable to in vivo experiments for studying tissue structure and physiological activity even in the deep regions with fluorometric techniques.