The contribution of cell death in regulating cellular populations of periodontal ligament was studied in young adult rats. Mandibular first molar periodontium was prepared for light-microscopic radioautography after a pulse of 3H-thymidine in 6 rats and for electron microscopy in 4 rats. The labeling index for 3H-thymidine and the density of fibroblast-like cells were computed from radioautographs. The percentages of dying or dead cells and macrophages were computed from electron micrographs. The labeling index of cells within 20 microns of bone and cementum was significantly lower (p less than 0.01) than the labeling index within the body of the periodontal ligament. The patterns of cellular density and indices of death were the inverse of the labeling indices. Macrophages were plentiful (% macrophages = 3.68% +/- 0.30) and were clustered around blood vessels (mean distance from blood vessel = 2.3 microns). However, only 10% of dying or dead cells were within 10 microns of blood vessels. These data show that death of cells in the periodontal ligament may, in part, balance production of cells by mitosis. The relationships between labeling index, index of death, and cellular density suggest that cells born in the middle of the periodontal ligament may migrate to regions of high cellular density near bone and cementum, and that they may die there. Macrophages do not appear to be associated with dying cells of the periodontal ligament.