The eye vesicle of Xenopus borealis has been replaced with the transplanted olfactory primordium from Xenopus laevis in an attempt to determine whether cells from the transplant could migrate along the regrowing olfactory nerve and become incorporated into the CNS of the host. The use of X. laevis and X. borealis pairs allowed us to distinguish the cells of the host from those of the donor at the cellular level by means of the characteristic fluorescent nuclear spots (Q bands) of X. borealis. Transplantation was performed on pairs of animals at stages 23/24. The olfactory anlage was readily incorporated into the host, often fusing with the host homolateral organ and inhibiting the regrowth of the eye vesicle. An olfactory nerve developed from the transplanted organ. In the majority of cases, the nerve reached the diencephalon at the level of entrance of the optic nerve. Along the nerve originating from the transplanted organ we observed a stream of cells with the characteristics of the donor. These cells penetrated the host's CNS and became incorporated into it. The nature of these cells has not been ascertained by specific neuronal markers. However, on the basis of their morphology and disposition, the hypothesis suggested is that some of the migrating cells are neurons.