In serum sickness, mechanisms by which circulating immune complexes become localized in the walls of vessels and glomeruli have been studied. In affected arteries, morphologic observations showed that circulating marker particles of carbon would rapidly deposit along the luminal surface of the internal elastic lamina. This, as in previous studies, suggested an increase in vascular permeability during which large molecules were capable of being trapped by a filtering membrane in the vessel wall. In attempts to prevent the increase in vascular permeability, rabbits were treated with antagonists of histamine and serotonin. Such treatment markedly inhibited the localization of immune complexes in glomeruli, the development of proteinuria, and glomerular endothelial proliferation. Cardiovascular lesions also were largely prevented from developing. Depletion of platelets, the principal reservoir of vasoactive amines, had a similar though less pronounced effect. While the deposition of immune complexes was inhibited, allergic inflammation in general was not, since normal rabbits treated as above were found capable of developing full Arthus reactions and acute nephrotoxic nephritis. Hydrodynamic factors were noted to be important in determining the location of arterial lesions. Studies of aortas from unmodified rabbits and from those with surgically induced coarctations of the abdominal aorta revealed intimal lesions concentrated at areas of high turbulence, such as at branches, bifurcations, outflows and zones of configurational change. Lesions in these areas were also largely inhibitable by depletion of platelets or by antagonists of histamine and serotonin.