The transplacental neurotoxicity of methylmercury (MeHg) on the fetal rat brain was studied. Adult female rats were administered 1, 2 or 3 mg/kg/day methylmercury chloride (MMC) orally for either 5 or 12 days, and were then mated. They were subsequently administered MMC in the same manner until the end of gestation. On embryonic day 22, a proportion of the fetal brains were histologically examined. Neuronal degeneration of varying degree was detected consistently in the brain stem, cingulate cortex, thalamus and cerebral basal area, including the hypothalamus. The distribution pattern of neuronal damage was different from those in rats treated with MeHg in the postnatal or adult stages. This finding suggests that pathomechanisms in MeHg intoxication operate distinctively in the fetal brain. The offspring derived from dams treated with 1 mg/kg/day MMC for 5 pregestational days and throughout pregnancy survived with inherent brain lesions. This experimental model could be a useful tool for research on the neurotoxicity of MeHg in the human fetal brain.