Neural nuclei from the presumptive neural area of late gastrula and the definitive neural plate regions of early neurula and mid-neurula embryos of Rana pipiens were transplanted into enucleated eggs of the same species. The results obtained confirm the previous findings, that in the course of neural differentiation there occurs a gradual progressive change which restricts the capacity of most neural nuclei to promote the normal development of test eggs. In the current report, serial transplantation of neural nuclei from the open neural-fold stage demonstrated that the nuclear condition responsible for these developmental restrictions is stable and cannot be reversed to a more normal expression by repeated exposure to test egg cytoplasm. Chromosome analyses of abnormal neural nuclear-transplants revealed two karyotypic groups of abnormal individuals. One group, comprising the majority of neural nuclear-transplants, possessed recognizable karyotypic aberrations. In some cases the chromosome abnormalities were severe and development ceased prior to overt cellular differentiation. In other cases, the karyotypic abnormalities were less severe and the transplant embryos developed into abnormal post-neurulae and larvae, which displayed variable patterns of cellular deficiencies not consistent with the neural origin of their nuclei. A second group which represents 3 of the 17 individuals whose chromosome constitution and cellular deficiencies were analyzed possessed the standard karyotype, and exhibited cellular deficiencies primarily in mesodermal and endodermal derivatives. This pattern of deficiencies is distinct from that found in embryos resulting from the transplantation of nuclei from endoderm cells and is consistent with the neural origin of the donor nuclei. The probability that the characteristic deficiencies seen in neural nuclear-transplant embryos are a phenotypic expression of the differentiated state of the donor nucleus is discussed.