Twenty-one pigs were poisoned with rations containing 611, 1000 or 2000 g. arsanilic acid per ton. The chief symptoms were tremor of the head and incoordination followed by progressive blindness, ataxia and paresis. The only post-mortem abnormality was distension of the urinary bladder. The principal histological lesions were degeneration of the optic nerves, optic tracts and peripheral nerves, but such lesions could not be detected until about four, 10 and 14 days respectively after the onset of symptoms. The arsenic levels in various tissues were estimated. Concentrations in liver and kidney cortex were highest in the early stages of illness, two to 14 days after the onset of symptoms.