Autologous immune complex-type nephritis was induced in Sprague-Dawley rats. The animals were killed when daily urinary protein excretion exceeded 300 mg (group 1) or 70–100 mg (group 2). Normal rats served as controls (group 3). Endocytosis of immune deposits by glomerular visceral epithelial cells was investigated by conventional and immunoelectron microscopy. In group 1, membranous glomerulonephritis was confirmed by conventional electron microscopy and immunofluorescence microscopy. By the former technique abundant endocytotic activity was shown in epithelial cells and by immunoelectron microscopy rat IgG was demonstrated in both the deposits and the epithelial cell phagosomes. Groups 2 and 3 were very similar and the presence of IgG could not be demonstrated. Results from group 1 strongly support the assumption that at least part of the phagosomes are derived from the deposits. Endocytosis seems to be dependent on the degree of membrane damage and the amount of immune deposit