Gradual distension of the knee joint with plasma was observed to result in weakening of the quadriceps muscle in sixteen subjects. In normal subjects this occurred before pain was experienced, but in some subjects with articular disease, particularly rheumatoid arthritis, pain preceded weakness of the quadriceps. In a patient with neuropathic disease of the knee joint muscle weakness did not develop even when the joint was distended at high pressure. Similarly, in other subjects, the instillation of local anesthetics delayed the appearance of weakness until the joints were distended at higher pressures. These studies suggest that stimuli from the knee joint reflexly inhibit the lower motoneurons supplying the quadriceps muscle. Reflex inhibition may lead to the muscle weakness, the atrophy, and the deformity that are seen in association with joint disease.