Connective tissue cells responding to wounding of the periodontal ligament of the lower first molar in mice were studied using the techniques of radioautography and grain counting. Animals were given an intraperitoneal injection of 2 uCi 1g3H‐Tdr 1 hr before being killed at either 30, 72 or 120 hr after wounding. The ligament in 1 um plastic sections was divided into compartments on the basis of distance from the wound, and the relative number of labelled cells in each compartment was assessed at 30, 72 and 120 hours after wounding. The distance of each labelled cell from the closest blood vessel was also measured at each time to detect relative movement of labelled cells away from blood vessels. In a Parallel experiment, haematogenous progenitors of macrophages were depleted by irradiating the animals with 800rads prior to woundilng to determine if mistakes in identification between fibroblasts and marophages could significantly affect the results. The ultrastructural characteristics of 150 of the3H‐Tdr labelled cells was examined in thin sections of wounded periodontal ligament prepared for elctron microcope radioautography, The majority of cells lebelled 30 hours after wounding were confirmed to be paravascular, and most of them were found to be located within 200 μ of the wound margin. Some of these cells appreared to have divided a number of times between 30 and 72 hours after wounding, and to have migrated into the wound between 70 and 120 hours after wounding. Examination of the irradiated material and the electron microscope radio‐autographs suggested that significant numbers of macrophages had not been included in the counts of labelled cells. The elctron microscope radioautographs also suggested that cells which exhibited different degrees of cytodifferentiation had incorporated