There have been a few studies on canine nephrotoxic glomerulonephritis produced by anti-glomerular basement membrane serum (AGBM), but these reports have not focused on an alteration in the charge properties of glomerular basement membrane (GBM). In this study, rabbit AGBM or normal rabbit serum (NRS) was given intravenously (2 ml/kg body weight) to 16 male beagle dogs. An alteration of anionic sites (ASs) of GBM was studied quantitatively using polyethyleneimine as a cationic probe by electron microscopy at weeks 1, 2, 4, and 8 postinjection. In AGBM-treated dogs, severe or mild proteinuria continued until week 2. At weeks 4 and 8, there was no significant difference in the intensity of proteinuria between AGBM- and NRS-treated groups. Until week 2 postinjection, there were significantly fewer ASs of GBM in AGBM-treated dogs than in NRS-treated dogs. At week 8, however, there was no difference in ASs of GBM between AGBM- and NRS-treated dogs. The fact that a reduction of glomerular AS occurred in AGBM-treated dogs with severe or mild proteinuria and the recovery of AS in the GBM coincided with an improvement of proteinuria suggested that alteration of the glomerular ASs might play an important role in the pathogenesis of proteinuria in canine anti-GBM nephritis.