Nickel is necessary for the biosynthesis of the hydrogenase, carbon monoxide dehydrogenase, and of factor F 430, found in a number of genera of bacteria. Urease from jack beans and several species of plants is also a nickel protein. These plant enzyme systems can affect animals via the microbiological digestion of food in the rumen. Nickel is a constituent part of all organs of vertebrates. Its absorption can be controlled. Low nickel offers reduce growth; this is particularly true of intra-uterine development. Such offers also decrease the life expectancy of reproducing animals. Nickel deficiency is accompanied by histological and biochemical changes and reduced iron resorption and leads to anaemia. It can disturb the incorporation of calcium into skeleton and lead to parakeratosis-like damage, which finds expression in disturbed zinc metabolism. Nickel deficiency results in lower activities of different dehydrogenases and transaminases and, above all, of alpha-amylase, and particularly affects carbohydrate metabolism. A marked decrease in metabolism was observed in the case of the energy sources fat, glucose, and glycogen. Nickel therefore performs a vital function in metabolism: it is an essential element. The nickel requirements of human beings and animals amount to less than 500 micrograms/kg and are probably even considerably lower. It therefore follows that, in view of the available nickel offer, primary nickel deficiency in human beings and animals can be excluded, at least in the present state of knowledge. On the other hand, it should be remembered that, 25 years after the discovery of the essentiality of manganese, this element was included among the trace elements of academic importance only, whereas today it is a feed additive.