Activation of protein kinase C (PKC) is implicated as an important mechanism by which diabetes causes vascular complications. We have recently shown that a PKC β inhibitor ameliorates not only early diabetes‐induced glomerular dysfunction such as glomerular hyperfiltration and albuminuria, but also overexpression of glomerular mRNA for transforming growth factor β1 (TGF‐βΙ) and extracellular matrix (ECM) proteins in streptozoto‐cin‐induced diabetic rats, a model for type 1 diabetes. In this study, we examined the long‐term effects of a PKC β inhibitor on glomerular histology as well as on biochemical and functional abnormalities in glomeruli of db/db mice, a model for type 2 diabetes. Administration of a PKC β inhibitor reduced urinary albumin excretion rates and inhibited glo‐merular PKC activation in diabetic db/db mice. Administration of a PKC β inhibitor also prevented the mesangial expansion observed in diabetic db/db mice, possibly through attenuation of glomerular expression of TGF‐β and ECM proteins such as fibronectin and type IV collagen. These findings provide the first in vivo evidence that the long‐term inhibition of PKC activation in the renal glomeruli can ameliorate glomerular pathologies in diabetic state, and thus suggest that a PKC β inhibitor might be an useful therapeutic strategy for the treatment of diabetic nephropathy.—Koya, D., Haneda, M., Nakagawa, H., Isshiki, K., Sato, H., Maeda, S., Sugimoto, T., Yasuda, H., Kashiwagi, A., Ways, D. K., King, G. L., Kikkawa, R. Amelioration of accelerated diabetic mesangial expansion by treatment with A PKC b inhibitor in diabetic db/db mice, a rodent model for type 2 diabetes. FASEB J. 14, 439–447 (2000)