The effects of each of several tannins purified from Rhei Rhizoma on serum constituents were investigated in rats with adenine-induced renal failure. Blood levels of urea nitrogen, methylguanidine (MG), and guanidinosuccinic acid (GSA) were significantly decreased in rats given ( – )-epicatechin 3-O-gallate at a dose of 2.5, 5, or 10 mg/kg body weight/day for 24 days. The creatinine (Cr) level was also significantly decreased in rats given 5 and 10 mg of this compound. A significant decrease in urea nitrogen, MG, and GSA was found in rats given 6.25 mg procyanidin B-2 3,3’-di-O-gallate. However, unlike the former two components, the administration of 12.5 mg of procyanidin C-1 3,3’,3”-tri-O-gallate produced a considerable or significant increase in blood levels of urea nitrogen, Cr, MG, and GSA. RG-tannin had a weaker overall effect on serum constituents except for GSA in comparison with the corresponding effect of ( – )-epicatechin 3-O-gallate and 6.25 mg of procyanidin B-2 3,3’-di-O-gallate. Rhatannin tended to increase the serum nitrogen constituents.