Electron microscopic observations were carried out on the spontaneous glomerular lesions in male APA hamsters from 3 to 12 months of age. Until 6 months of age, focal expansion of mesangial region due to an increase of matrix material and mesangial cells was characteristic, and segmental thickening of capillary basement membrane and partial effacement of foot processes of podocytes were also sometimes observed. At 12 months of age, although all of these changes became more severe, the most prominent alteration was found in podocytes, which showed various degenerative changes. No deposition of amyloid fibrils was detected in any portion of the glomerulus.