The effect of methylmercury chloride (MMC) on the developing rat nervous system was studied by light microscopy. Rats on postnatal day 2 (P2), P15 and P60 were administered 10 mg/kg/day MMC orally for 10 days. In newborn (after P2) rats, there was no abnormal activity or body weight loss. Young (after P15) rats showed weight loss on the 9th day after starting MMC, and subsequently unsteadiness, gait disturbance and paroxysmal convulsions appeared. In adult rats, weight loss began on the 6th day after starting MMC, and the hind-limb crossing phenomenon was induced on the 13th day. Histopathologically, minimal damage was found in the hippocampus and brainstem in newborn rats. In young rats, widespread neuronal degeneration was observed in the cerebral neocortex, CA3 and CA4 regions of the hippocampus, neostriatum, red nucleus, and various brainstem nuclei. In adult rats, neuronal damage was most extensive in the cerebellum and spinal dorsal nerve roots. These findings indicate that neuronal vulnerability to MMC exposure differs depending on the postnatal developmental stage and the brain region in the rat.