AbstractBone resorption, stimulated by injection of parathyroid extract, was measured in vivo in microphthalmic mice as the rate of release of 3H from bone after incorporation of 3H‐proline. Bone resorption in these mice, which inherit osteopetrosis, was less than 10% of that in normal littermates. Autoradiography confirmed the reduction in removal of radioactive bone matrix and in bone growth in microphthalmic mice. The ability of these mice to raise the serum calcium concentration in response to PTE was also reduce. These results, that bone resorption is reduced in microphthalmic mice, are discussed in relation to the pathogenesis and cure of the disease.