Many substances are suspected to be uremic toxins, and harmful effects have been confirmed for most of them, whereas some have been found to have no adverse effects on the body. Attention has long been focused on terminal metabolites, including urea, creatinine and uric acid, as uremic toxins. However, the relationship between these substances and uremic symptoms is now considered to be somewhat minor, and the noxious effects of other uremic toxins, e. g. guanidino compounds, have been clarified. In the present experiment, the effects of a green tea tannin mixture and its individual tannin components on methylguanidine were examined in rats with renal failure. The green tea tannin mixture caused a dose-dependent decrease in methylguanidine, a substance which accumulates in the blood with the progression of renal failure. Among individual tannin components, the effect was most conspicuous with (-)-epigallocatechin 3-O-gallate and (-)-epicatechin 3-O-gallate, while other components not linked to gallic acid showed only weak effects. Thus, the effect on methylguanidine was found to vary among different types of tannin. In addition, the effects of green tea tannin in nephrectomized rats were examined. There was an increase in blood urea nitrogen, serum creatinine and urinary protein and a decrease in creatinine clearance in nephrectomized control rats, whereas better results for these parameters were obtained in rats given green tea tannin after nephrectomy, demonstrating suppressed progression of renal failure. When the renal parenchyma was partially resected, the remnant kidney showed a decrease in the activity of radical scavenger enzymes. Green tea tannin, however, was found to lighten the kidney under such oxidative stress. Mesangial proliferation and glomerular sclerotic lesions, which were conspicuous in rats that were not given green tea tannin after nephrectomy, were also relieved.