The VX-2 carcinoma in 32 New Zealand white rabbits was studied morphologically with respect to the osseous effects induced by the innoculation and subsequent growth of tumour in the soft tissues over periods ranging from 1 to 5 weeks. Although more severe changes were noted in bones of tumor-bearing than in non-tumor-bearing limbs, effects could be seen in both experimental situations. Severe bone marrow hyperplasia was noted consistently in tumor-bearing animals. A marked increase in numbers of osteoclasts over control rabbit bones was observed along the surface of bones near and distant from the tumor. Resorptive changes were also noted in the cortex of tumor-bearing rabbits. The increase in osteoclast numbers was related chronologically to the development of hypercalcemia and was proportional to the degree of hypercalcemia at the time the animals died. The number of osteoclasts was inversely proportional to the serum creatinine. A diminution of osteoclast numbers was noted in the latter stages of the disease. These changes may be the morphologic expression of humoral hypercalcemia accompanying the VX-2 carcinoma.