Little is known of the clearance mechanism of blood in the joint and it is still controversial. In order to clarify the possible mechanism of the clearance, rabbits own venous blood was injected into their own right knee joints of twenty-four normal adult rabbits and the opposite left knee joints served as control. The animal were killed at 12 hours, 1, 2, 3, 4, 7, 14 and 21 days after intra-articular injection of autologous blood. For histological and histochemical studies, the fresh synovial tissues were obtained from infrapatellar regions. The sections were stained by the alcian-blue (pH 2.5) for mucopolysaccharides and methylgreen pyronin for ribonucleic acid (R.N.A.). And finally, for metachromasia the sections were stained with toluidine blue (pH 2.5). In hematoxylin-eosin stained preparation, the synovial cells did not show any gross alterations in the early stages of experiment, but there was an increasing infiltreation of some inflammatory cells into the synovial and subsynovial tissues. Many erythrocytes were found lying free between the synovial cells, in the synovial matrix and subsynovial tissues. Any synovial change that shows a patchy thickening of the synovium due to increase in both the size and number of synovial cells and villous proliferation, became marked at the fourth day after experiment. On the seventh day villous proliferation were still seen but persist in an attenuated form. These changes converted to nearly normal from two weeks after experiment. With methyl-green pyronin stain for R.N.A., the synovial cell showed a definite increase in number in pyroninophilic granules and in intensity of staining reaction at two days of hemarthrosis and these findings were seen even in the third week specimens. After 12 hours of experiment, the synovial cells and the matrix showed a quite clear positive reaction with alcine-blue. And these changes showed a most strong positive reactions at 24 hours of experiment. But after 24 hours the synovial cells gradually showed a decreasing tendensy in intensity of staining reaction to alcian-blue. Since the fourth day of experiment the synovial and subsynovial tissues were closer to normal with alcian-blue. Toluidine blue gave only faint metachromasia to the very rarely in the deeper layer of synovium. We confirmed some of the injected erythrocytes escaped from the joint into the synovium and were cleared from the joint, and blood cells are clears from the joint by phagocytosis of synovial cells. It is suggested that these changes correlates with various histological and metabolic changes of the synovial membrane following a experimental hemarthrosis.