Removal of the occipital cortex in newborn rats results in the rapid and nearly complete degeneration of the dorsal lateral geniculate nucleus (dLGN) in 5 days. In previous studies we have shown that transplants of embryonic posterior cortex neurons, which are allowed to develop in culture for 5 days prior to transplantation into the site of the lesion, prolong the survival of a particular population of host dLGN neurons for an additional week. In this study we tested the possibility that the transplant cells synthesize diffusible proteins which are responsible for this neurotrophic effect. Culture medium conditioned by explants of embryonic occipital cortex and diencephalon was concentrated by vacuum dialysis or ultrafiltration through membranes with at least a 10-kDa cut-off. This concentrated medium was loaded into polyacrylamide or sodium alginate gels which were then implanted into the cavity of the lesion. Five days after implantation, the alginate-conditioned-medium implants result in a 3-fold increase in dLGN survival compared to unconditioned medium controls, while a two-fold increase in survival of the nucleus is found with the polyacrylamide-conditioned-medium implants. Proteolysis of the conditioned medium eliminates all neurotrophic activity. The results suggest that the death of dLGN neurons following the cortical lesion is due to the loss of diffusible proteinaceous neurotrophic factors--factors that may operate during normal in vivo development of the geniculocortical pathway.