1. By the use of a device that allows movement of a human finger joint to take place uninfluenced by muscle activity, the coefficient of friction has been determined between living articular surfaces.

2. The force of friction rises as the range of movement is increased, because of the tension then developing in the ligaments and the soft tissues surrounding the joint.

3. Measurements have also been made of the forces of friction within the ankle joint of the dog and within four types of reciprocating bearings (plastic, oil-lubricated, "floating" and hydrostatic).

4. By altering the load borne by the joints and bearings it has been shown that joints resemble in their behaviour those bearings in which a film of fluid is maintained between the fixed and moving surfaces, rather than bearings depending wholly or in part on boundary lubrication.

5. It is suggested that joints normally owe their great freedom of movement to a special type of fluid film lubrication that has been termed "weeping lubrication," supplemented by "floating lubrication," though on occasions boundary conditions may prevail.

1. By reducing the viscosity of the synovial fluid within the ankle joints of rabbits and then subjecting these to prolonged exercise, wear and tear of articular cartilage can be consistently produced.

2. This finding is an indirect confirmation of the view that fluid film lubrication is an important factor in the mechanical efficiency of joints.

3. The special properties of synovial fluid and articular cartilage that allow fluid film lubrication to exist within joints that are, in effect, slowly moving, heavily loaded, reciprocating bearings are discussed. They account for the remarkable resistance to wear and tear exhibited by synovial joints under physiological conditions.