The olfactory bulb of an embryo (BALB/c strain mouse) was transplanted into the neocortex or hippocampal formation in a young female adult host (AKR strain mouse) and matured for at least 1 month. Projection fibers from the transplant were demonstrated immunohistochemically using the allelic form of the Thy-1 system between both mice. Fibers from the olfactory bulb transplanted into the frontal cortex showed non-specific elongation in two directions where normally there are no targets of the olfactory bulb fibers: one was toward the cortical surface among radially oriented host fiber systems and another on the corpus callosum running rostrocaudally together with the host's long association bundle. Transplants placed in the hippocampal dentate and hilar regions emit fibers mainly towards the dentate molecular layer and these fibers spread medially and laterally within the molecular layer. The results suggest that projection neurons (mitral and tufted cells) in the transplanted olfactory bulb have the potential ability of non-target-directing axon elongation into the host nerve tissue which usually prevents the penetration of newly growing fibers.