Antimony (Sb) is a useful metalloid in many industries and a therapeutic agent for leishmaniasis in developing countries. Thus, it is expected that humans and wild animals face the risk of exposure to Sb. Although Sb is generally recognized as a toxic element, the mechanisms underlying its toxicity have not been fully elucidated yet. The objective of this study was to evaluate species differences in Sb distribution in blood and urine between rat and hamster. Antimony was more preferably accumulated in rat red blood cells (RBCs) than hamster RBCs. On the other hand, it has been reported that arsenic is bound to a specific cysteine residue in rat hemoglobin, which results in the substantial accumulation of arsenic in rat RBCs. These have led us to formulate the hypothesis that Sb, which belongs to the same group in the periodic table as arsenic, is also accumulated in the same manner as arsenic. However, because Sb was less accumulated than arsenic even in rat RBCs, Sb seemed to have less affinity for the cysteine residue than arsenic. Trivalent Sb showed greater accumulation than pentavalent Sb in rat RBCs. Consequently, species differences in Sb distribution between rat and hamster could be attributed to the affinity for the specific cysteine residue in hemoglobin.