Chronic methylmercury exposure in the monkey (Macaca mulatta). Behavioral test of peripheral vision, signs of neurotoxicity, and blood concentration in relation to dose and time.

Small daily doses of methylmercury hydroxide were administered to rhesus monkeys for periods of up to 17 months. Behavioral tests of peripheral vision and of the accuracy and rapidity of hand movements did not disclose any early subtle deficits preceding the onset of obvious signs of neurotoxicity. These signs appeared suddenly and involved reduced food intake (anorexia), clumsiness of jumping, loss of fine control of the digits, and uncoordinated mastication. With a constant daily dose of 0.1 mg/kg or less, blood concentration of mercury reached a peak after about 2 months, and then decreased to about half the peak value. Subsequently, increasing the daily dose level above 0.1 mg/kg (range of 0.12-0.21 mg/kg) produced an increase of blood concentration which tended to stabilize in the range of 2.0-2.5 ppm. After several months at these elevated concentrations all animals exhibited signs of neurotoxicity.