A 13-week repeated oral dose toxicity study of madder color (MC), a natural food colorant extracted from the roots of Rubia tinctorum L., was performed using F344 rats. Five groups of animals, each consisting of 10 males and 10 females, were fed diet containing 0, 0.6, 1.2, 2.5 or 5.0% MC for 13 weeks. During the experiment, lower body weight was evident from the 2.5% dose. Hematologically, fluctuation in red blood cell (RBC) parameters suggestive of weak anemia (females), and slight increases of platelet counts (both sexes) and white blood cell (WBC) counts (males) were observed at higher doses. Serum biochemically, slight fluctuations were observed in many parameters, including increased total protein (TP), conjugated bilirubin, Ca, and inorganic phosphate, and decrease of the albumin/globulin (A/G) ratio in both sexes, with dose-dependence for TP and A/G from 0.6% in females. Histopathological changes were mainly observed in the renal proximal tubules, such as microvesicular vacuolar degeneration in the cortex and karyomegaly in the outer medulla involving both sexes, lesions being evident even with 0.6%. In the outer medulla, elevation of cell proliferation activity as assessed with proliferating cell nuclear antigen was observed in males from 2.5%. Severity of focal necrosis of hepatocytes was increased only in females at 5.0%, while the increased relative liver weight as with the increased conjugated bilirubin was evident in both sexes from 1.2%. The results thus suggest that MC exerts mild toxicity, targeting liver, kidneys, and possibly RBCs and WBCs, some renal changes being evident from 0.6% in diet, that is attributable to be the lowest-observed adverse effect level (305.8-309.2mg/kg body weight/day).