Rapid transient changes in axial transmission of near infrared light through the outer segments of retinal rods of Bufo marinus are induced by illumination. The reasons for these changes are not clear. The changes in optical transmission may be useful in the study of photoreceptor function. However, the study of photoreceptor functions through the use of indicator dyes may be confounded by the intrinsic light-induced changes of optical properties of the photoreceptor cells.