Glomerular epithelial slit alterations and their relation to proteinuria have not been studied in detail in New Zealand Black/White (NZB/W) mice. The kidneys of proteinuric and nonproteinuric female NZB/W mice and normal Swiss albino mice were perfusion-fixed with tannic acid-glutaraldehyde and studied by light and electron microscopy. Semiquantitative studies were performed on full montages of glomeruli enlarged 10,000 times. Fine structural alterations of the epithelial slits, with emphasis on the slit diaphragm, were studied on semiserial thin sections. Proteinuric NZB/W mice with features of membraneous nephropathy exhibited: (1) wedging of electron-dense deposits below the slit diaphragm, (2) enlargement and distortion of interpedicel spaces, (3) displacement, folding, and stacking of slit diaphragms, (4) formation of occluding junctional complexes in residual slits, and (5) variable loss of foot processes. Similar alterations were not observed in controls or nonproteinuric NZB/W mice, including animals having complexes inglomerular mesangia but not in epithlialslits. These studies show that in NZB/W mice, abnormal protein excretion is associated with structural modification of the slit pore and suggest a role for such a component in the process of protein ex