Recent evidence indicated that methylmercury (MeHg) contaminated rice can be a significant source of MeHg human exposure, but the health implications are not known. The objective of this study was to study the kinetics, speciation, and effects of MeHg contaminated rice using a rat model. Five groups of adult Sprague-Dawley rats (n = 10 in each group) were fed control rice, low (10 ng/g MeHg) and high (25 ng/gMeHg) MeHg contaminated rice. Two groups of the positive control were fed control rice spiked with the same levels of MeHgCl. Shortterm exposure to low level of spiked MeHgCl stimulated the growth of male rats while long-term exposure to spiked MeHgCl inhibited the growth in female rats. There was no temporal variation of total mercury (THg) concentrations in the rat fecal samples from each group, and the THg concentrations significantly correlated with the inorganic Hg concentrations in the feeding rice. There were significant differences in the accumulation of THg and MeHg among different groups and different organs. THg and MeHg concentrations in the kidney were the highest among the organs examined. The blood and brain had high percentages of THg as MeHg, which indicates that MeHg can easily pass through the blood-brain barrier and has a high affinity for brain tissue. Exposure to rice containing 25 ng/g MeHg decreased antioxidant function and damaged the nervous system in rats, but no significant effects were found in the group fed with rice containing 10 ng/g MeHg. MeHgCys in rice is less toxic than spiked MeHgCl to rats. The toxicity of MeHg both decided by its concentration and speciation.