The volume and DNA synthesis of the neuroepithelium in induced myeloschisis in Long-Evans rats as shown by hematoxylin-eosin and BrdU/antiBrdU immunohistochemical staining patterns were examined at different stages of embryonal development. On day 14 of gestation, control animals contained BrdU-incorporating cells mainly at the alar plate of the closed neural tube. On the same day, the everted neural plate of rats with myeloschisis showed active, diffuse uptake of BrdU in the cells of the matrix layer, although an increase in the volume of the everted neural plate was not yet identifiable. On day 21 of gestation, rats with myeloschisis showed a marked increase in the volume of the neuroepithelium compared with controls. Our investigations suggest that, in myeloschisis, more neuroepithelial cells than normal retain their capability for DNA synthesis on day 14 of gestation, and the overgrowth of the neuroepithelium found on day 21 is possibly a secondary effect of failure of neural tube closure.