During the course of experiments designed to study synaptic relationships between the terminals of retinal axons and the various cell populations in the optic tectum of the frog, Rana pipiens, we found that neurons in many of the retinorecipient nuclei, including the tectum, are labeled transneuronally following injections of horseradish peroxidase into the optic nerve. In the optic tectum, particular cell groups are labeled to the extent that their dendrites as well as their somas are filled with reaction product while other cell types, which, on the basis of the location of their somas or dendrites, seem equally likely to receive direct retinal projections, remain free of label. Electron microscopic investigation of the optic tectum reveals that the label is confined to pre- and postsynaptic processes. These results suggest that the transneuronal transport depends on a transfer of horseradish peroxidase from presynaptic terminals to postsynaptic cells rather than on a widespread diffusion of the enzyme through the neuropil followed by a selective uptake by particular cell groups. These results also suggest that only some of the tectal cell groups which receive direct retinal projections may be transneuronally labeled. The transneuronal transport of horseradish peroxidase is useful since it reveals the morphology as well as the location of at least some of the retinorecipient cells. Moreover, the robust nature of this phenomenon makes the frog a good choice for future studies of the mechanism of transneuronal transport.