The utilization of3H‐uridine (3H‐U) was used to assess age related changes in RNA biosynthesis by connective tissue cells of the periodontal ligament. One hundred forty‐four BNL mice from 5 to 78 weeks old were injected with 5μ Ci/gm body weight of3H‐U and killed from 15 min to 30 days later. Autoradiographs were prepared from 5μm thick, decalcified, sagittal sections of the maxillae. Labeled periodontal connective tissue cells were grain‐counted (1) below epithelial attachment, (2) below crevicular epithelium, (3) above alveolar crest, and (4) at the alveolar crest.3H‐U incorporation was apparent at all time periods (15 min to 30 days). Nuclear labeling achieved peak values 1–4 hours after3H‐U injection. Cytoplasmic values were maximum at 1–2 days. Nuclear and cytoplasmic counts were similar at 8–16 hours. From 5 to 52 weeks of age, peak values were progressively reduced, curves were broadened and shifted towards later time periods. This age decline in the rate and magnitude of incorporation of3H‐U reflects changes in the cellular needs for RNA and protein synthesis necessary for matrix production and cellular maintenance. Some age changes were seen at 26 weeks of age. In the oldest animals (78 weeks), there appeared to be an arrest in the age‐decline in3H‐U utilization. This was probably associated with the inflammatory changes reported in the periodon