W e have previously reported that LEC (Long Evans Cinnamon) rat with spontaneous hepatitis and hepatoma is a very useful animal model for analysis of the pigmented teeth caused by bilirubin. In this study, to elucidate the mechanism of bilirubin pigmentation in LEC rat, we compared the LEC pigmented teeth with the experimental pigmented teeth caused by bile duct ligature and exogenous bilirubin administration to LEA rat, isolated from LE (Long Evance) closed colony accompanied with LEC rat, and we examined the chronological serum bilirubin and the pathological analysis of the pigmented teeth. By means of bile duct ligature of LEA rat, serum bilirubin level increased to 8.9 ± 1.1 mg/dl at the peak, but that of the pigmented teeth was not observed (0/9). As a result of this, intraperitoneal bilirubin was administered (14 mg/kg/day, for 4 days) in addition to bile duct ligature of LEA rat. Thus, serum bilirubin level increased to 13.7 ± 1.4 mg/dl at the peak and bilirubin pigmentation of the dentin was observed in 5 of 10 (5/10) rats, but there was no significant difference between the serum bilirubin level of the pigmented group and that of the non-pigmented group. Histopathological analysis of the LEA pigmented teeth using optical microscope showed that pigmentation of LEA rat was observed in incisors as a stripe in the dentin and the stripe ran parallel to the incremental line similar to that of LEC rat, and that the abnormal dentin tube was observed in the pigmentation area, but there was no correlation between abnormal dentin tube and teeth pigmentation. The microradiographic analysis disclosed enhanced permeabilities of the pigmented areas of all 5 rats, but the permeabilities of all 5 non-pigmented rats did not change. These results suggest that the hypocalcification of the matrix of the teeth is closely concerned with the bilirubin pigmentation in addition to the high level serum bilirubin.