In order to elucidate the effects of NPY directly on bone tissue, two different doses of NPY (NPY dose 1= 1X10-5 M and NPY dose 2 = 1X 10&amp / #65533 / 6 M) and NPY dose 2 plus its inhibitor were applied together with hyaluronic acid (HA) into the intramedullary area of right tibia of Wistar rats. HA alone was administered as the control group. On three time points, day one, week one and week two after administration, the tibiae were collected and stored at &amp / #65533 / 20oC for analysis. Evaluation was performed via conventional radiography, dual energy X-ray absorbtiometry (DEXA), quantitative computerized tomography (QCT), three point bending test (TPB) and histology techniques. QCT was used to assess both atomic content and density of both medulla and cortex of tibiae. From DEXA results, it was observed that inhibition of NPY causes an increase in the bone mass from first day to second week. This phenomena was also observed in histology results so that new bone formation in the inhibitor administered bone was encountered at week two. In both medulla and cortex areas&amp / #65533 / atomic content, an increase in average effective atomic number was displayed after administration of NPY plus NPY inhibitor throughout two weeks. In addition, density of medulla of tibiae measured by QCT also revealed an increase in bone mass when inhibitor is applied throughout two weeks. As a result, overall evaluation of data obtained from DEXA, QCT and histological analysis revealed that NPY inhibits bone formation or have a pro-osteoclastic effect / inversely HA displayed osteogenic effect.