The alterations of actin filaments in the glomerular epithelial cells of adriamycin-induced nephrotic rats were studied by electron microscopy and immunohistochemistry using both of the peroxidase-antiperoxidase and avidin-biotin methods. After the administration of adriamycin, the glomerular epithelial cells showed cell swelling, deformation of cell shape, vacuole formations, retraction or fusion of foot processes, detachment of the plasma membrane from the glomerular besement membrane (GBM), and an increase of cellular organelles. In parallel with these alterations, abnormal distribution of intracytoplasmic microfilaments of 5-6 nm in diameter was observed, which formed clusters, particularly gathering densely along the plasma membrane of the fused foot processes that abutted the GBM. Immnohistochemically, actin was demonstrated on the site of the above described filaments. The above results suggest that actin filaments may closely relate to the morphological maintenance of the glomerular epithelial cells and to their function as well.