Alterations of tubules and glomerules have been reported previously in kidneys of rat neonates after aminoglycosides were given to the mother during gestation. Here, we have studied the effects of gentamicin on the development of the glomerular basement membrane (GBM). Pregnant Wistar female rates were treated with gentamicin. Deliveries occurred normally. Using electron microscopy, we looked at the deepest glomerules of the kidneys of 1-day-old neonates: myeloid bodies were found in podocytes, and the GBM appeared thicker and denser than in controls. Anionic ferritin, injected intravenously crossed the GBM in prenatally gentamicin-exposed animals, but not in controls. Furthermore, urine electrophoresis showed the presence of proteins normally found only in the urine of fetuses 2 days before birth. We suggest then, that in utero exposure to gentamicin leads to a delay of renal maturation and that the GBM is altered in juxtamedullary nephrons while it is normally differentiated and functioning in controls. Thus exposure to drugs before birth could be harmful to the GBM.