In addition to acting as a source of the amino acid itself, methionine can act as a source of methyl groups. The requirement for sulphur amino acids is therefore influenced by the inclusion level of alternative methyl donors in the diet e.g. choline. An initial experiment was designed to ensure an adequate supply of choline so that in subsequent studies the need for methionine per se could be established. Blood metabolites were used to study the requirements. Three diets with different levels of methionine plus cystine (3.5, 4.5 and 5.5 g/kg) were formulated and randomly assigned to 36 pigs. To a basal diet, containing 660 mg/kg choline, choline chloride was added producing diets with choline levels of 660, 860, 1060, 1260, 1460 and 1660 mg/kg. The pigs were randomly allocated three levels of choline, each for a period of three days; this ensured that each treatment was repeated six times. Blood samples were taken by jugular venepuncture 4 hours after feeding at the end of each 3 day period and repeated at 3 liveweights (35, 55 and 75 kg). Blood parameters (urea and plasma amino acids) were used to measure the response.