This study focused on the glomerular structural changes observed after cyclosporine A (CsA)-induced nephrotoxicity. Renal structural changes were examined in rats treated with oral CsA, given as a daily dose of 50 mg/kg for periods of up to 49 days. By means of scanning electron microscopy and morphometry, we first demonstrated that the most conspicuous and reproducible ultrastructural changes could be detected in the endothelial cells of the glomerular capillaries. These changes included a reduction in the fenestral pore size and partial disappearance of endothelial fenestration, the appearance of microvilli-like projections on the endothelial surface, and flattening and widening of the cytoplasmic folds. We believe that the ultrastructural changes observed in this study are partially responsible for the alterations in renal function seen in the cyclosporine A-treated model, and that these alterations are caused by CsA-induced vasospasm.