1. At ligament-bone junctions in dogs existing at varying levels of physical activity, strength diminished as activity diminished.
2. At the proximal end of the tibia, subperiosteal resorption weakened the ligament-bone junction of the medical collateral ligament. A similar change was seen in the fibular attachment of the lateral collateral ligament but not in the other ligament attachments about the knee.
3. Microscopically, widespread subperiosteal bone resorption developed in inactive dogs.
4. Simple caging for six weeks or more produed boen resorption.
5. With continued caging, bone resorption at the ligament-bone attachment healed over a period of six months or more as fibrous tissue replaced resorbed bone and then became mineralized. The widespread subperiosteal bone loss consequent to inactivity probably healed in the same way, suggesting on the basis of this histological evidence that inactivity produces bone resorption which is followed by bone accretion.