The presence and possible roles of cytoplasmic contractile elements in glomerular epithelial podocytes and the glomerular mesangium are briefly discussed. Glomerular podocytes contain actinlike filaments distributed throughout their cytoplasm and more concentrated filamentous material within their foot processes. Antibody labeling and nitrobenzoxadiazole-phallacidin labeling have confirmed the presence of especially high concentrations of actin within podocyte foot processes. Studies with cytochalasins have suggested that contraction of actin within podocytes leads to a flattening of foot processes and a loss of the intervening filtration slits. Conversely, relaxation of these contractile elements leads to a narrowing of the bases of foot processes and an increase in the number of fully open intervening filtration slits. These observations have led to the proposal that glomerular podocytes have the potential to regulate the glomerular filtration rate by changing the shapes of their foot processes and thereby decreasing or increasing the number of filtration slits available for solute efflux across the glomerular wall. There is also evidence to indicate the presence of contractile elements within the glomerular mesangium. These contractile elements may be represented in part by bundles of fine filaments associated with electron-dense bodies that are present within many processes that radiate from these cells. In vitro studies in particular have suggested that these cells may possess contractile properties. It has been proposed that contraction of the mesangial cells may lead to a shunting of blood within the glomerulus or to a decrease in glomerular size.