Experimental allergic angitis induced by two separate intravenous injections of horse serum antigen into rabbits was studied by immunofluorescent procedures. Free specific antibody persisted in the periarterial connective tissue of the heart, liver and speen up to 21 days after primary immunization. Shortly after the booster shot, Ig, C3 and antigen were simultaneously and intensely and intensely present in the intimal layer of small arteries and in the periarterial connective tissue of the heart and liver. This finding indicates that the inducing antigen is trapped by the preexisting specific antibody and local Arthus's reaction follows at this site. From 7 to 10 days after the booster shot, periarterial infiltration of inflammatory cells and panarteritis with fibrinoid degeneration were observed in these organs. It was considered that panarteritis was not observed in the spleen because the splenic structure allows leakage of the injected antigen via a peripheral central artery into the marginal azone. Therefore, antigen-antibody reaction in the periarterial region may not have been conspicous. The author concludes that the deposition of circulating immune complexes in the intimal layer of arteries and the occurrence of local Arthus' reaction in the periarterial connective tissue were responsible for the development of panarteritis observed after the booster shot by inducing injury from both the luminal and periarterial sides.