The purpose of this study was to determine the shape, size, and spatial configuration of the fibroblust progenitor compartment (PC) of the tooth‐related periodontal ligament (PDL). Five albino rats (weight 200 gm) were killed 1 hr following intraperitoneal injection of 3H‐thymidine. The incisors were decalcified, embedded in glycol‐methacrylate, and sectioned serially, perpendicularly to the long axis of the tooth. Ten 2‐μm‐thick sections along each tooth were selected, processed autoradiographically, and the distance from the apex measured. Labeled and unlabeled fibroblasts were counted in 8 × 80‐μm consecutive layers. Cell density (CD) and labeling index (LI) were plotted on a three‐dimensional system of coordinates depicting the directions: apex to incisal edge, cementum to bone, and cemento‐enamel junction (CEJ) to lingual tooth surface. The CD along the tooth was constant, except at 2–6 mm distant from the apex, where it was significantly higher. The highest concentration of cells around the tooth was in the 20‐μm region adjacent to the cementum. The PC on the mesial arid lateral tooth sides extended 1–4 mm from the apex, with the LI peak (6.5%) occurring at 2 mm. The PC on the lingual tooth side was located at 2‐G mm from apex, peaking (14%) at 4 mm. On the cemenuim‐bone coordinate, the PC extended to about 50 μm from the cementum on all sides, the LI peak appearing at a distance of 20–40 μm from the tooth. The LI was consistently higher on the lingual side than on the mesial and lateral aspects.