The aim of this study was to assess whether developing cerebral cortex produces diffusible factors that can affect the growth of thalamic cells and, if so, what the role of these factors might be during the formation of thalamocortical connections. We studied interactions between cultured organotypic explants from mice maintained in defined serum-free medium. First, we cultured explants of embryonic dorsolateral thalamus in isolation from any other tissue; after culture, these explants were viewed intact and then sectioned. We estimated the numbers of healthy and pyknotic cells before and after culture, and the rates of mitosis in the explants during culture (using bromodeoxyuridine). Based on these data, we concluded that the majority of cells in the thalamic explants survived, although significant numbers of pyknotic cells did accumulate. Thalamic explants extended either very few or no neurites when cultured alone. We then cultured explants of embryonic thalamus near to explants from other tissues. A gap was always maintained between the explants, and we measured the length and density of neurite outgrowth from each thalamic explant. Slices of embryonic cortex promoted a small but significant increase in the amount of growth from thalamic explants. Postnatal cortex stimulated much more profuse neurite outgrowth; postnatal cerebellum had less of an effect, and postnatal medulla or liver had none. We showed that there was significantly more outgrowth from thalamic explants cultured in medium that had been preconditioned with cortical slices than from thalamic explants cultured in control medium, confirming that diffusible factors were produced by the cortex. The survival and mitotic rates of thalamic cells were unaffected by co-culture with the cortex. We conclude that the developing cortex releases diffusible factors that stimulate the growth of thalamic neurites and that other regions of the brain may also release the same substance(s). The lack of a specific source of thalamic growth promoting factor(s) argues against a role for these factors in guiding thalamic axons to specific targets; indeed, we were unable to demonstrate any chemotropic guidance of thalamic axons towards cortical explants in collagen gels. Since postnatal cortex has a more potent stimulatory effect than prenatal cortex, it seems possible that, in vivo, the cortical-derived factors act mainly on thalamocortical axons that have located their targets and are in the process of arborizing and refining their connections.