By injection of staphylococcus aureus or 3% silver nitrate solution into the tympanic cavities through the ear drum, otitis media was induced in 48 rabbits. Changes of size of capillary nets and caribres. of capillaries were observed in the rabbit's ear suffered from otitis media for 24 hours to 130 days.The conclusions summarized were as follows:1) In the staphylococcus group, the capillaries in the mucous membrane of the tympanic cavity revealed a remarkable change. In the silver nitrate group, the flow of India ink to the capillaries was hardly recognized, and major area of the, tympanic cavity was in the state of necrosis.2) The capillaries of the tympanic cavity wall rapidly increased in number in early stage (5_??_20 days) of middle ear inflammation, and remained increased even in later stage (1_??_4 months).3) New vessels grown from conical germ were extremely fine. India ink did not extravasate through the vessel wall.4) The calibre of new vessels gradually grew larger and finally became as large as that of ordinary capillary network. By this time swelling of germ diminished or disappeared.5) The calibre of venules grew larger rapidly and remarkably since very early stage (2 days). In some animals, they grew about twice as large.6) The calibre of arterioles also grew larger in early stage, but was not so remarkable as venules.7) The calibre of arterioles and venules in 1_??_4 months had tendencies that their rate of dilatation became lower than those in early stage.8) The net capillaries were slow-in dilatation compared with that of arterioles and venules, and the rate of dilatation was lower than that of arterioles. In 3_??_4 months, there were tendencies that the rate of dilatation became higher.9) During the course of this experiment, no remarkable change of new vessels and the calibre was revealed in the bony substance.10) The leakage of India ink was remarkable in the venous side of capillaries, suggesting the increase of permeability or haemorrhagia per diapedesin in these areas.11) In the staphylococcus group, the growth of granulation tissue from the tympanic cavity wall was observed in almost all cases in 7_??_130 days. The granulation tissue was observed mostly on the surface of the malleus, the stapes, the incus, the round window and the drum, in order of their frequency. In the silver nitrate group, the granulation tissue was observed in all cases in 10_??_50 days.12) There were two types of growth of the blood vessels in the granulation tissue-one grown from germ, and another from fibroblasts around the area of bleeding.13) In the staphylococcus group, in 5 cases out of 23, and in the silver nitrate group, in 7 cases out of 8, remarkable dilatation of capillaries in the stria vascularis was observed, mostly in early stage of middle ear inflammation.14) In the inner ear, a remarkable dilatation of venules was also shown.15) Net capillaries in the sria vascularis showed the remarkable dilataion compared with those in the middle ear. Dilatation was also remarkable in the crista acustica.16) Spread of inflammation to the inner ear was mostly through the round window. Dilatation of the inner ear vessels in early stage was probably due to inflammatory products permeating through the round window vessels.17) In the staphylococcus group, in 4 cases out of 23, the growth of secondary cholesteatomain the auditory canal were observed (in one case, cholesteatoma invaded into the tympanic cavity).The capillaries surrounding cholesteatoma were dilated forming dense network.