Serial renal biopsies were performed to observe in detail the age‐related glomerular IgA deposition in ddY mice. Moreover, lymphocytes from spleens and Peyer's patches (PP) were immunologically examined in the mice at 10 months of age. The incidence of IgA deposition in the glomeruli was 0% at 3 months, 33% at 7 months, 60% at 10 months and 100% at 13 months, but IgA deposits were less intense than IgG and IgM deposits. Serum levels of IgA and polyclonal IgA responses of PP cells were significantly higher in the mice with glomerular IgA deposits. However, neither the Lyt‐l+/Lyt‐2+ cell ratio nor the polyclonal IgA responses of spleen cells were related to IgA deposition. Therefore, gut‐associated lymphoreticular tissue (GALT) probably plays an important role in the pathogenesis of the high IgA response and glomerular IgA deposition observed in this strain. These results suggest that ddY mice would be valuable as a model of spontaneous glomerular IgA deposition, as well as being useful for studying the possible relationship between the mucosal immune system and glomerular IgA deposition. ACTA PATHOL JPN38: 1