Bovine joints were oscillated under the maximum possible static load in a device which measured their coefficients of fraction while the joints were running. Joint wear was evaluated by gross and histologic inspection. Joints oscillating under loads, just below the structural capability of the joints, did not show significant wear even after long periods of time. When periodic impact loading was added to this regimen, cartilage wear became rapid and was easily discernible. The implications of these findings to the wearing away of articular cartilage are discussed.