Samoyed hereditary glomerulopathy (SHG) in dogs resembles hereditary nephritis (HN) in man. Affected males and carrier females spontaneously develop proteinuria, but only males progress to renal failure. We examined the evolution of splitting of glomerular capillary basement membranes (GCBM) in affected male and carrier female dogs. At birth, examination by light microscopy (LM) in all dogs showed normal glomerular capillaries, but by electron microscopy (EM), many GCBM showed separation of endothelial from visceral epithelial cell basement membranes. By LM, glomerular capillaries of affected males appeared normal for up to 4 months, but then isolated and eventually all capillaries became thickened and split. However, by EM, a bilaminar appearance of the GCBM was seen within 1 month which evolved into multilaminar splitting that became extensive as renal failure ensued. Carrier females showed mild nonprogressive changes in GCBM. These results with SHG permitted speculation on the evolution of splitting of GCBM in human HN.