OBJECTIVE: Comparing histological changes according to the level of dextrose-concentration of proliferant under the same osmolarity on Achilles tendon of rat. METHOD: One millimeter of three proliferant solutions (20% dextrose water-group A, 5% dextrose water mixed with NaCl-group B, NaCl solution-group C) with the same osmolarity (1, 110 mOsm) was injected around the right Achilles tendon of each rat, whereas the left was not injected to be used as control. After six weeks of injection, the injected tendons and controls were obtained. The transverse diameter of gross specimen, the count of fibroblasts on light microscope, and the findings of cross-sectional analysis using electron microscope were compared. RESULTS: Overall, transverse diameter and the count of fibroblasts increased in the injected specimens compared to controls, however, their significant differences were demonstrated only for the two groups injected with dextrose containing solutions (p<0.05). However, A and B groups did not show significant differences in all parameters investigated. On electron micrograph, fibril diameters of solution-injected tendon consisted of either extremely large or small sizes with the limited intermediate sizes. CONCLUSION: Although high osmolar solution could increase the transverse diameter and fibroblast counts, however, dextrose-containing solution was much more effective as a proliferant solution.