Infection with the abomasal nematode, Ostertagia ostertagi, is an important cause of impaired productivity in young cattle in temperate parts of the world. Such losses have been associated with marked changes in feed intake, gastrointestinal function, protein, energy and mineral metabolism, and in body composition. The reduction in feed intake is an important factor in the pathogenesis of infection and may account for a large part of the difference in weight gain between ad libitum fed control and infected calves. Despite the obvious importance of inappetance, only recently has an association been made between reduced intake, altered gut motility and elevated levels of certain gastrointestinal hormones, such as gastrin. It has been suggested that the elevated gastrin levels accompanying abomasal parasitism may impair reticulo-ruminal motility and slow down abomasal emptying, leading to a stasis of ingesta and a reduction in feed intake. The rise in blood gastrin levels may also be partly responsible for the marked hyperplasia of the fundic mucosa seen in abomasal infections. Pronounced changes in protein metabolism have also been associated with Ostertagia infection. Radioisotopic studies have demonstrated increased losses of albumin into the gastrointestinal tract which are accompanied by an increase in the rate of synthesis in the liver. Dietary protein breakdown in the abomasum is also likely to be impaired, although there is evidence of a compensatory increase in protein digestion in the lower gut of parasitised calves. Increased losses of albumin are not always accompanied by increases in faecal nitrogen, suggesting that albumin is broken down and recycled as ammonia. Radioisotopic studies in animals with intestinal nematode infections have demonstrated a marked reduction in muscle protein synthesis and an increase in protein synthesis in gastrointestinal tissue. Such changes in the balance of protein synthesis are likely to be brought about by alterations in the balance of certain metabolic hormones. Marked changes in energy metabolism also accompany Ostertagia infection. Parasitised calves exhibit a marked increase in non-esterified fatty acid levels, resulting from the mobilisation of adipose tissue, and a reduction in digestive efficiency of energy, probably associated with the increase in cycling of protein through the gastrointestinal tract and the compensatory increases in protein synthesis. Mineral metabolism may also be affected although relatively little work has been conducted in cattle. Changes in body composition reflect a reduction in deposition of muscle protein and fat, and an increase in bone content and water retention.