We studied the effects of the long-term and small-dose administration of methylmercury chloride (MMC) on the cerebral function in rats. MMC, at a dose of 0.7 mg/kg/day, was subcutaneously injected for 85 consecutive days in nine adult male Sprague-Dawley rats. They were then sacrificed on the final day of exposure (MMC group) after both completing observations on behavioral changes and also determining the local cerebral glucose utilization (LCGU) as an indicator of the cerebral neuronal activities. Histological examinations of the brain and the sciatic nerve were also performed. In addition, seven rats who received physiological saline also served as a control. LCGU significantly decreased in the visual cortex, lateral geniculate nucleus and medial geniculate nucleus without any accompanying histological alterations. Severe axonal degeneration of the sciatic nerve was also observed, which corresponded to the previously described crossed leg phenomenon. The present results suggest that the damage to the peripheral nerve was much more severe than that to the brain, which caused behavioral changes. Although no cerebral morphological changes were observed, brain dysfunction showed a selective involvement of the visual and auditory systems. This finding suggests that LCGU is a sensitive method for detecting the subclinical cerebral dysfunction caused by long-term and small-dose MMC intoxication in the rat brain.