1. The rate of disappearance of intra-articularly administered 133Xe from the knee joint was studied in normal subjects and in patients suffering from various arthritides. The disappearance curve was monoexponential and could be described by a biological half life . The half lives were shown to be reproducible, but could be reduced marginally by aspirating knee joint effusion when present.
2. It was demonstrated that the values depended, not upon the pathological diagnosis, but upon the degree of inflammatory involvement of the knee joint at the time of study.
3. The effect of intra-articularly administered hydrocortisone upon the value was investigated in twenty-five rheumatoid subjects. The mean value obtained before injection of hydrocortisone was significantly lower than the mean value obtained 24 hr later. Significant clinical improvement was also noted. The relationship between individual clinical improvement and the change in value was examined.