The effects of a high dose of cobalt-60 radiation on rhesus monkey mandibles have been studied with light microscopy. The radiation was similar in amount and method of delivery to that given human patients with cancer. Adult rhesus monkeys were irradiated with 4,500 rads of cobalt-60 radiation in 10 fractions over 12 days, the equivalent of 7,000 rads given in 35 fractions over 7 weeks. Observations ranged from 1 week to 6 months postirradiation. Clinical observations paralleled the signs and symptoms noted in human patients receiving radiation therapy to the head and neck. Osteocytes were killed only in the direct path of the radiation beam in the outer lamellar and Haversian bone. The medullary area of the irradiated mandibles underwent marked proliferation of new bone. The periodontal ligament became more densely collagenized, lost the organization of its principal fiber groups, and demonstrated changes in the vessels of the interstitial spaces. The Haversian canals showed changes ranging from obliteration of the blood vessels to complete plugging of canals with osteoid. Within the field of irradiation, the periosteum exhibited a loss of cellularity, loss of vascularity, loss of osteoid formation, and a tendency to be easily stripped from the bone during processing. A few areas showed exophytic bone growth and normal-appearing periosteum. The marrow of the irradiated animals showed striking changes, including fibrosis, proliferation of new bone, and obliterative endarteritis of many of the blood vessels.