Glomerular mesangial cells of the rat kidney contain actin, nonmuscle myosin, tropomyosin, and the muscular Z-line protein, alpha-actinin. This was shown for actin, myosin, and alpha-actinin by immunoblotting as well as by immunoelectron microscopy. Tropomyosin was localized in mesangial cells by immunofluorescence. In cultured mesangial cells, actin, myosin, and alpha-actinin constitute a considerable amount of the total cellular protein contents. In mesangial cells in situ actin, myosin and alpha-actinin were found to be colocalized within conspicuous microfilament bundles that traverse the cell body or major processes in various directions and project into either the tonguelike pericapillary processes, which run toward mesangial angles, or into the microvilluslike lateral extensions that abut on the perimesangial portion of the glomerular basement membrane (GBM). Thereby, the GBM of opposing mesangial angles as well as of opposing portions of the perimesangial GBM are regularly interconnected by filament bundles within mesangial cells that contain actin, myosin, and alpha-actinin. The authors suggest that the major function of actin-, myosin-, and alpha-actinin-containing filament bundles in mesangial cells is to create an isometric tension (or minute isotonic contractions) to counteract the distending forces of the rather high intracapillary hydraulic pressure and its resulting pressure gradients across the capillary wall and across the perimesangial GBM.