Immunofluorescence studies of rat kidneys from 3-24 mo. of age demonstrated deposition of immunoglobulins, predominately IgM, within the glomerular mesangium by age 3 mo. Immunoglobulins eluted in acid buffer, did not fix complement, were not associated with inflammatory changes, and increased markedly with the onset of proteinuria at about 12 mo. Rats 24 mo. Old also had mesangial deposits of IgG and fibrin. No basement membrane deposits were seen, and autoantibodies to antigens in normal kidney, liver, spleen, and skeletal muscle were not demonstrable in serum or eluates of kidneys from aged animals. The focal and segmental glomerular sclerosis which develops in aged rats does not appear to be mediated by glomerular deposition of auto-antibody or immune complexes. Mesangial accumulation of macro-molecular material, perhaps as a consequence of the age-associated increase in glomerular permeability, may contribute to the development of the glomerular sclerosis of aging in the rat by impairing mesangial phagocytic or clearing mechanisms rather than through immunologically mediated tissue injury.