The specificity of trophic interactions in the rat visual system is investigated in vivo by using a combination of tissue culture and CNS transplantation methods. In a companion paper (Repka and Cunningham: '87) we showed that explants of embryonic day 14 (E14) occipital cortex are biased to contain different cortical cell populations depending on whether the explants develop in culture with diencephalon or with optic tectum. In this study we transplanted these precultured cortical explants into the cavity created by a lesion of the occipital cortex in newborn rats and then measured the neuron-occupied volume and the numbers of thymidine-labeled cells in the surviving ipsilateral dorsal lateral geniculate nucleus (dLGN) of the host rats. The results were compared to animals with lesions but no transplants, animals with transplants of E14 cortical tissue that had not been precultured, and animals with cerebellar transplants that had been similarly precultured either with other cerebellar tissue or with diencephalon. At 5 days postlesion, both the largest dLGN volume and the greatest number of labeled dLGN neurons survive in animals with cortical transplants precultured with diencephalon or other cortex. The surviving dLGN neurons that are rescued by these transplants are generated on E15 or E16, a period that corresponds to the latter part of geniculate neurogenesis. Relatively few cells generated on E14 survive in any group of animals. Furthermore, animals with all types of cortical transplants have significantly larger volumes of surviving dLGN than animals with either lesions only or cerebellar transplants.(ABSTRACT TRUNCATED AT 250 WORDS)