The spinal central canal area in the early postnatal rat differs markedly in appearance from that in the adult. Dorsally and ventrally, the walls of the canal lack nuclei and appear to be composed of processes from cell bodies at some distance from the lumen. Additionally, a densely packed cluster of cells similar to those in the lateral walls is situated ventral to the canal area. Preliminary data indicated that the cells in this cluster were extremely susceptible to ionizing radiation; the present study: (1) characterizes these cells ultrastructurally in the normal animal; and (2) examines radiation-induced changes. Ultrastructurally, the cells forming this cluster in the normal 3-day-old rat were tightly packed, and their outlines followed the contours of adjacent cells. The cytoplasm, which contained an abundance of organelles, including distinct, wide-bore endoplasmic reticulum with darkly stained cisternae, occasionally surrounded bundles of axons. Synapses and junctional complexes were absent. Both nuclear and cytoplasmic characteristics are those of immature astrocytes. By 6 h following irradiation of the 3-day-old, many cells in this cluster were pyknotic, and by 24 h the cluster had essentially disappeared. The number of cells surrounding the lumen decreased less rapidly than in the cluster. By 10 days post-irradiation (P-I) this number was approximately 39% less than in the control. These changes indicated a differential sensitivity to ionizing radiation between the cells of the ventral cluster and those elsewhere in the central canal area.