Some effects of parasitism, endotoxaemia or sepsis can be mitigated by provision of extra protein. Supplemented protein may encompass a metabolic requirement for specific amino acids (AA). The current study investigates a method to identify and quantify the amounts of AA required during inflammation induced by an endotoxin challenge. One of each pair of six twin sheep was infused in the jugular vein for 20 h with either saline (control) or lipopolysaccharide (LPS, 2 ng/kg body weight per min) from Escherichia coli. Between 12 and 20 h a mixture of stable isotope-labelled AA was infused to measure irreversible loss rates. From 16 to 20 h all sheep were supplemented with a mixture of unlabelled AA infused intravenously. Blood samples were taken before the start of infusions, and then continuously over intervals between 14 and 20 h. At 20 h the sheep were euthanised, and liver and kidney samples were taken for measurement of serine-threonine dehydratase (SDH) activity. LPS infusion decreased plasma concentrations of most AA (P < 0·05; P < 0·10 for leucine and tryptophan), except for phenylalanine (which increased P = 0·022) and tyrosine. On the basis of the incremental response to the supplemental AA, arginine, aspartate, cysteine, glutamate, lysine (tendency only), glycine, methionine, proline, serine and threonine were important in the metabolic response to the endotoxaemia. The AA infusion between 16 and 20 h restored the plasma concentrations in the LPS-treated sheep for the majority of AA, except for glutamine, isoleucine, methionine, serine and valine. LPS treatment increased (P < 0·02) SDH activity in both liver and kidney. The approach allows quantification of key AA required during challenge situations.Key words: Endotoxin lipopolysaccharide: Amino acids: Irreversible loss rate: Serine-threonine dehydratase: SheepThe protein and amino acid (AA) demands of animals for normal physiological states (e.g. growth, pregnancy and lactation) are now well characterised, especially for pigs and poultry (1) and with progress made in ruminants (2) . These requirements are altered, however, in response to injury, infection and parasitic challenge. In many cases, impact of these challenges can be ameliorated and the rate of recovery improved by provision of additional protein in the diet (3)(4)(5)(6) . Often, however, the amount of protein required is not known, with consequences for both animal performance and farm economics.More importantly, protein per se may not be required, but instead the metabolic responses to the challenge may increase the demand for specific AA. For example, practical and theoretical arguments have been presented for additional arginine in cardiovascular and pulmonary disorders (7)(8)(9)(10) , for cysteine (11) , leucine (12) and threonine (13) in sepsis, for glutamine in endotoxaemia (10) , and for phenylalanine to support acutephase protein synthesis (14) . In other situations, combinations of AA have been proposed (10,15,16) . Obviously different stages of injury, infection or recover...