In order to evaluate the toxicity of Np237 in a large animal, a neptunium citrate complexed solution was administered intravenously in doses up to 12 mg (8.3 µc) per kg of body weight to 1-year-old sheep. Deaths were observed in all the animals administered 12 mg of Np237/kg, and in four out of five animals that received 6 mg/kg. Liver function was impaired, as determined by I131 rose bengal blood clearance, in animals which received doses as low as 1.5 mg (1.0 µc) per kg of body weight. Gross hemorrhages were evident in the livers of the group administered 6 and 12 mg/kg. Histopathological changes observed included neuronal damage in the brain, together with parenchymal damage in the liver and kidneys. The microscopic lesions observed generally resembled those resulting from heavy metal toxicity.