Functional, histological and immune-histological examination were performed in altogether 64 Wistar-rats, in order to control the effect of a therapy with 2 mg/kg per body weight indomethazine lasting 2 months at the model of an experimental immune complex nephritis. In 44 rats after presensibilisation an immune complex nephritis was performed by intraperitoneal injections with human serum albumin which were repeated three times a week. 24 glomerulonephritis animals and other 20 animals without glomerulonephritis were daily administered indomethazin through a tube probe, the remaining 20 animals with glomerulonephritis served as untreated control groups. The excretion function of the kidney was tested before the beginning of the experiment, 2 weeks after the beginning of the therapy and the regular serum injections, respectively, and before the end of the experiment by determination of the biological half-life period of 131J-hippuran. In every case one day before this the proteinuria during 24 hours was determined. At the end of the experiment the kidneys were examined histologically and immune-histologically. The results showed that indomethazin does not lead to a clear influence on the proteinuria in the immune complex nephritis of the rat. The excretion of 131J-hippuran was significantly restricted, whereas the histological and immune-histological preparations in the animals with foreign serum injections showed clear changes of the glomeruli in the sense of an early stage of the immune complex nephritis, however, they did not show any essential influence by indomethazin. That is, indomethazin had altogether no favourable effect on the immune complex nephritis of the rat.