In a murine lupus model, the kidneys of 42 MRL mice of different age groups (substrains MRL/Mp-lpr/lp and -+/+) were studied by light and transmission electron microscopy. The results demonstrate the ultrastructural feature of the mesangial-proliferative glomerulonephritis which is one of the observed forms of glomerulonephritis. The most important alterations are a severe proliferation of mesangial cells (with an increase in rough endoplasmic reticulum and mitochondria) and electron dense deposits in different sites of the glomerular basement membrane (subepithelial, subendothelial, intramembranous). These deposits are proposed to be immune complexes. Osmiophilic intracytoplasmic inclusions of the mesangial cells are indications of an altered renin production. Alterations of the glomerular epithelial cells are characterized by fusions of the epithelial pedicles, an increase of microvilli and intracytoplasmic concentrations of electron dense material which are also proposed to be immune complex deposits. The morphological feature of the mesangial-proliferative glomerulonephritis is completed by an activation and focal edema of endothelial cells. The described alterations are discussed and compared with findings in other species.