A dense network of NPY immunopositive fiber terminals was found in the retinal recipient and less in number in the periventricular fiber layers of the optic tectum. Axons of immunostained cells in the pretectal area, and torus semicircularis could be followed to the tectum. Eye removal did not influence tectal NPY immunopositivity, while a pretectal lesion markedly reduced this immunoreaction ipsilateral to the lesion. Lesions of the torus semicircularis did not affect tectal NPY immunopositivity. It is concluded that the main source of NPY immunoreactive fibers is the posterior thalamic nucleus in the pretectal region, while the periventricular NPY immunopositive fibers may be of intrinsic origin.