The level of melatonin in the frog (Rana tigrina regulosa) retina was studied at midlight and middark in a 12L:12D cycle and under different lighting conditions. It was found that the frog displayed a diurnal rhythm of melatonin in the retina with high levels in the dark period. When the animal was subjected to an extended dark period, the level of retinal melatonin was significantly (P less than 0.05) increased. In addition, the normal low level of retinal melatonin in the light period was significantly (P less than 0.05) increased after dark treatment, and the normal high level of retinal melatonin in the dark period was significantly (P less than 0.05) lowered following light exposure. These results suggest that synthesis and secretion of melatonin in the frog retina is controlled by environmental lighting. This supports the hypothesis that melatonin plays an important role in the regulation of the photomechanical changes of eye pigmentation, an important element in the control of light sensitivity and acuity in the eyes of vertebrates. Moreover, these findings suggest that the photoreceptor may be a neuroendocrine or neurohumoral transducer which transduces the environmental lighting into neuroendocrine or neurohumoral secretion, melatonin.