Thymidine-H3 radioautography was used to study the proliferative response to penetrating wounds of the mouse spinal cord. In one group of animals mononuclear leukocytes which infiltrate nervous tissue wounds were labeled by injecting thymidine-H3 prior to injury. In two other experimental groups the cells which synthesized DNA in the nervous tissue following the injury were labeled by giving either a single injection of isotope shortly before sacrifice or by giving four injections during the 24 hours prior to sacrifice. The animals were sacrificed over a five day period following spinal cord injury. Although the labeled nuclei in all three groups were similar in appearance, their distribution about the lesion was very different. The labeled blood cells were greatly concentrated at the wound, while the cells that responded to injury by DNA synthesis were much more evenly spread throughout the tissue. When these distributions were converted to straight lines and compared statistically, there was a very low probability that the group of cells labeled before injury and the two groups labeled after injury were samples from the same population. Although mononuclear leukocytes do proliferate in and around nervous tissue wounds, other cells originally present in the nervous tissue wounds, other cells originally present in the nervous tissue must also proliferate.