1. Joint compliance estimations were made on the knee joints of fourteen patients with rheumatoid arthritis, three patients with osteoarthritis and three healthy volunteers.

2. Diminishing values for capsular compliance were found in the normal volunteers, the patients under the age of thirty-five years with rheumatoid arthritis, the patients with osteoarthritis and the patients over the age of forty years with rheumatoid arthritis respectively.

3. In the young adult normal volunteers the average compliance was 8·1 cubic centimetres per millimetre of mercury at an intra-articular pressure of 5 millimetres of mercury and 0·7 cubic centimetre per millimetre of mercury at 50 millimetres of mercury. In the older patients with rheumatoid disease the average compliance was 2·1 cubic centimetres per millimetre of mercury at 5 millimetres of mercury and 0·4 cubic centimetre per millimetre of mercury at 50 millimetres of mercury.

4. Both the effects of ageing and the effects of disease appeared to be responsible for the low capsular compliance observed in patients over the age of forty years with rheumatoid arthritis, but duration of disease did not seem to be a contributing factor.