Occasionally forages accumulate nitrates in quantities that are toxic to some farm animals. Under certain environmental conditions, some plants may accumulate high concentrations of nitrates. The factors that can influence the accumulation of nitrates in the plants are drought, shade, use of herbicides, and application of nitrogenous fertilizers. Nitrates are not very toxic, but they are readily converted by bacteria in the alimentary tract into nitrites, which are much more toxic. In cattle and sheep this conversion takes place in the rumen and in horses in the cecum. Nitrites pass easily from the gastrointestinal tract into the blood, where they combine with hemoglobin in the red blood cells to form methemoglobin, a compound that is incapable of taking up and transporting oxygen (Fig. 1). Consequently the clinical signs of nitrite poisoning are those associated with oxygen deficiency and include general weakness and a fall in blood pressure. Death may follow from asphyxia. Young animals and babies are particularly at risk because the small volume of blood that they contain requires only a small amount of nitrite to convert all the hemoglobin to methemoglobin. Pregnant animals that are affected, but do not die, may abort later.