We examined the histological findings and cytoarchitectonic alterations in the rat spinal cord following matrix cell degeneration caused at different developmental stages, from neural plate formation through neuroblast generation. Ethylnitrosourea (ENU) 20 mg/kg body weight was administered transplacentally to the fetuses on the 10th embryonic day (E10) to 14th. The observations were made until the 21st postnatal day. Normally, mitoses were present scatteredly in the matrix cell layer of the neural plate or neural tube on E10 or E11, and gradually restricted to the dorsal portion of the alar plate as development occurred. The localization and number of degenerative cells as well as the site and degree of neuronal decrease in the completed dysgenetic spinal cord seemed to correlate with the topography and frequency of the mitoses in the matrix cell layer at the time of ENU administration. Disorder in the pattern of cytoarchitecture of neurons was not observed. The degree of hypoplasia of the white matter was proportional to the intensity of decrease of the spinal neurons. Aberrant myelinated fibers were not seen. No reactive gliosis, fibrosis or abnormal vascularization was observed at any time.