This paper summarizes our current knowledge of the structural details and probable functional significance of the system of contractile filaments in the glomerular mesangium. The description is based mainly on studies of superficial glomeruli of the rat kidney. The contractile apparatus of mesangial cells consists of microfilament bundles located predominantly within mesangial cell processes. The thickest microfilament bundles occur in the juxtacapillary mesangial cell processes, which directly abut the glomerular capillaries. The effector structure of mesangial cell contractility is the GBM. Mesangial cell processes are connected to the GBM either directly or through the interposition of extracellular microfibrils. In general, the contractile system of the mesangium interconnects opposing parts of the GBM. This arrangement is particularly obvious in the juxtacapillary processes, which underlie a mechanical connection between the GBM at the two opposing mesangial angles of a single capillary. The geometry and structural composition of the contractile apparatus of the mesangium indeed suggest a static rather than a dynamic function. In conjunction with the GBM, the mesangial contractile apparatus seems capable of supporting sufficient wall tension to counteract the distending forces acting across the capillary walls; the apparatus also seems capable of directly balancing the distending forces on the perimesangial walls. Assuming that mesangial cells are capable of isotonic contractions, the effect of such a contraction on capillary diameter and, consequently, on filtration area would be small.