The effect of osmium tetroxide, which is used in treatment of rheumatoid arthritis, was histologically and electronmicroscopically examined in the normal synovial membrane of the rabbit. The animals were albino rabbits, and they were 20 altogether. 0.3 ml. of 1% OsO(4) solution was injected, i.a., in the right knee. The left knee joint served as a control. One minute after the injection, black reduced osmium was observed in the synovial membrane. The endothelial layer of the synovial membrane coagulated and disappeared within some days. Deep in the synovial membrane, a foreign body granuloma and a connective tissue scar appeared in five weeks. The result was a strong thickening of the synovial membrane.