We have previously identified a population of serotonin-like immunoreactive (5-HT-ir) retinal ganglion cells in Rana pipiens. In this study, we examined serotonin-like immunoreactivity (5-HTLI) in a probable target of those cells, the optic tectum. We observed both 5-HT-ir fibers and cell bodies in this structure. 5-HT-ir cells were located in the cellular layers of the tectum, layers 2, 4, and 6, and scattered in its superficial layers. 5-HT-ir fibers in the tectum displayed a laminated organization and were located in tectal layers 3, 5, 6, 7, and 9. Retrograde labelling experiments showed that 5-HT-ir retinal ganglion cells projected to the optic tectum. However, these experiments also demonstrated that serotonergic neurons in the midbrain tegmentum, the nucleus isthmi, and the medulla did so as well. 5-HT-ir fibers seen in lamina A of layer 9 were very much reduced in density in animals in which the optic nerve had been lesioned for 3-6 months. Immunoreactive fibers in lamina B of layer 9 were not affected by the lesion. Our results suggest that 5-HT-ir fibers in lamina A of layer 9 are mainly of retinal origin, whereas those in lamina B originate from other brain areas. The 5-HT-ir tectal cells located in the cellular layers probably contribute the 5-HT-ir fibers seen in layers 3, 5, 6, and 7.