1. Uptake of intravenously administered radioactive technetium ( 99m Tc) was measured over the knee joints in normal human volunteers, in patients with osteoarthritis and in groups of synovectomized and unoperated patients with rheumatoid arthritis. The uptake was compared with clinical indices of inflammation (pain, tenderness swelling and stiffness), and the clearance rate of intra-articularly injected radioactive xenon ( 133 Xe). The 99m Tc uptakes were found to be unrelated to the isotope dose and the day-to-day reproducibility was acceptable. 2. The mean uptake of 99m Tc was within normal limits in osteoarthritis. Both in synovectomized and in unoperated rheumatoid arthritis 99m Tc uptake was significantly higher than in normal subjects. 3. Of the clinical indices studied significant correlation of 99m Tc uptake was found with pain and swelling in all groups of patients studied. 4. Faster clearance of 133 Xe in unoperated rheumatoid arthritis correlated well with the higher 99m Tc uptakes. 5. The results confirm that 99m Tc uptakes are raised in inflammatory arthritis but not in degenerative arthritis. The relation of 99m Tc uptake to the clinical indices of inflammation and to the 133 Xe clearance from the joint is discussed.

1. The rate of disappearance of intra-articularly administered 133 Xe 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. 1. The rate of disappearance of intra-articularly administered 133 Xe 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. 1. The rate of disappearance of intra-articularly administered 133 Xe 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. 1. The rate of disappearance of intra-articularly administered 133 Xe 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. 1. The rate of disappearance of intra-articularly administered 133 Xe 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.