To determine whether alpha-smooth muscle (alpha-SM) isoactin is expressed in the maturing kidney as well as the changes associated with maturation, we processed for immunocytochemistry kidney sections from Wistar-Kyoto rats at various prenatal (15, 17, 19, and 20 days) and postnatal (2, 5, 10, 15, and 90 days) ages using a monoclonal anti-alpha-SM actin antibody. At 15 days of gestation, only a few mesenchymal cells contained alpha-SM actin. However, other fetal vasculature structures (heart, aorta, peripheral blood vessels) expressed alpha-SM actin. Vascular localization was first observed at 17 days of gestation in larger corticomedullary vessels. As maturation progressed, actin expression accompanied the outward growth and branching of the kidney vasculature. During fetal life (17 days), alpha-SM actin also was expressed within juxtamedullary glomeruli. As the centrifugal maturation of nephrons proceeded, intraglomerular expression extended to outer cortical glomeruli. After 10 days of postnatal life, once glomerular development was completed, intraglomerular expression was no longer present. Peritubular capillaries expressed alpha-SM actin during early (fetal and neonatal) development, but not in the adult kidney. We conclude that 1) expression of alpha-SM actin in the developing kidney is delayed with respect to other vascular beds, 2) expression of alpha-SM actin follows the centrifugal pattern of nephrovascular development, and 3) glomerular and peritubular capillary expression of alpha-SM actin is a transient developmental phenomenon associated with active glomerulogenesis and capillary growth.