Groups of 5 male monkeys were fed 0, 0.01, 0.03, 0.1 and 0.3mg/kg/day of methylmercury chloride (as Hg) for 52 months. In the 0.3 and 0.1mg/kg/day groups, toxic signs including characteristic neurological ones appeared at 62 and 181 days (2.0 and 6.0 months) on average, respectively, during which period the total amounts of the test material ingested were 15.9 and 15.3mg/kg as Hg, respectively. Four animals from each group died or were sacrificed when they became moribund between the 57th and 70th days (1.9 and 2.3 months), and the 173rd and 242nd days (5.7 and 8.0 months), respectively.All animals of both groups had Hg exceeding 100ppm in the hair at the time of onset of toxic signs. On the day of sacrifice, the Hg level of the whole blood was 12.37 and 8.04ppm, and that of the red blood cells was 26.45 and 17.26ppm on average for the groups given 0.3 and 0.1mg/kg/day, respectively. Histopathologically severe lesions of the cerebral cortex in the occipital lobe, particularly in the striate area, extending to the neighboring cortex were the most prominent feature. The lesions in the 0.1mg/kg/day group were more extensive and frequent, and more advanced in terms of tissue destruction than those in the 0.3mg/kg/day group, probably due to the protracted course of poisoning in the former group. No alteration was observed in the cerebellar cortex and white matter, spinal cord and peripheral nerves. Tubular degenerations predominant in the proximal tubules of the kidney were also ascribed to the feeding of the test material. Dose-related Hg accumulations were found in the central nervous system, in which the cerebrum showed the highest levels of Hg, nearly 100% as methyl Hg. Higher Hg accumulations which did not appear to be dose-related were present in the liver and kidney.One animal from each of the 0.3 and 0.1mg/kg/day groups, to which the administration of the test material was suspended on the 151st and 541st days (5.0 and 17.8 months), respectively, survived the whole experimental period with complete or partial recovery from clinical signs. Histopathologically, however, extensive or localized cortical lesions persisted in these two animals. The high Hg levels in the hair at the time of suspension of administration of the test material decreased to the control level on the 644th and 1094th days (21.2 and 36.0 months), respectively. Although the Hg levels in various organs were very low, they were still higher, especially in the animal in the lower dose group, than those of the control, and the mercury contained was exclusively inorganic.The animals of the 0.03 and 0.01mg/kg/day groups survived the experimental period without revealing any clinical signs, except a reduction in body weight gain in the former group at the later stage. The total amounts of the test material ingested in the groups were 39.6 and 13.2mg/kg as Hg, respectively. In the hematological examinations no specific change was observed. In the blood biochemical examinations, there was no significant change, except elevation of urea-N after 30 months in the 0.03mg/kg/day group. The Hg levels in the hair in the 2 groups decreased gradually after reaching the peaks of 154.6ppm on the 1094th day (36.0 months) and of 63.4ppm on the 917th day (30.1 months), respectively. Similarly the Hg levels in the blood decreased gradually after reaching the respective peaks of 1.422 and 0.474ppm at 30 months. No histopathological change was found in organs including the nervous system. Dose-related Hg accumulations, much lower than those in the groups given 0.3 and 0.1mg/kg/day, were found in various organs except the kidney. The ratio of methyl Hg to total Hg was fairly high in the cerebrum, liver, spleen, and muscle, whereas it was very low in the cerebellum and kidney, especially in the former.