Locally administered 9Ds-hydroxy-10,12(E,Z)-octadecadienoic acid (9-HODE) caused a drastic inflammatory response in the experimental model of granulation tissue formation of the rat according to RUDAS (Arzneimittelforsch. 10,226-229, 1960). Three days after implantation of the polyvinyl chloride rings the granulation tissue became inhomogeneous with proliferation islets surrounded by edematous regions containing a diminished number of cells. The number of polymorphonuclear leukocytes and of macrophages was greatly enhanced in the whole tissue, whereas the number of lymphocytes was reduced. After seven days the whole granulation tissue was loosened, and its mass was twice as high as in the control animals. The number of fibroblasts per area unit and the hydroxyproline content were diminished. Linoleic acid and 13Ls-hydroxy-9,11(Z,E)-octadecadienoic acid (13-HODE) caused also some changes in the formation of granulation tissue, but in a different manner, in particular, without accumulation of polymorphonuclear leukocytes and macrophages, indicating the specificity of the effect of 9-HODE. The recruitment of leukocytes was not due to a direct chemotactic action of 9-HODE as shown in an agarose diffusion test comparing the effects of 9-HODE and leukotriene B4. The possible biological importance of the proinflammatory effect of 9-HODE is discussed.