A case of arsenic poisoning in cattle is reported, in which six out of fifteen animals died. The source of poisoning was traced to a substance in the loft of the hay-barn, which probably consisted of the remains of a pesticide which had been there for several years. This had become liquefied as the result of a leak in the roof and had found its way into the hay. The arsenic content of the substance ranged from 17 to 37 per cent, that of the hay being up to 0.3 per cent. The concentration of arsenic in the organs, blood, faeces and urine of the animals were indicative of poisoning. The main clinical findings consisted in profuse diarrhoea, loss of appetite, muscular weakness and an ataxic gait. Dehydration set in later and the animals became deeply soporous. Symptomatic treatment was mainly directed against dehydration, hypoglycaemia and hypocalcaemia. The greater part of the animals gradually recovered. Marked hyperaemia of the abomasal mucosa and, to a less extent, hyperaemia of the small and large intestines was observed on post-mortem examination.