The experiments described here were set up (a) to investigate the effect of age and (b) to investigate the effect of giving five diets which varied in methionine and choline or betaine contents on some of the enzymes that metabolize these nutrients in chick liver. Growth and carcass composition of the chicks fed on the different diets were also examined. There was no obvious relationship between age and enzyme activity in young chicks. Only a diet low in methionine (but not one low in choline) showed a significant decrease in growth and a change in carcass composition. The effects of diet on enzyme activity were complex. Choline oxidase (EC 1.1.3.17) activity was affected by the level of choline in the diet, being high when choline was present at high levels, especially when methionine was limiting. 5-Methyltetrahydrofolate homocysteine methyltransferase (EC 2.1.1.3) had a high activity in the livers of chicks fed on a conventional diet compared with those given semi-purified diets. Other enzymes showed minor changes in response to the diet. The diet low in methionine showed a lower activity of cystathionine psynthase (EC 4.2.1.22) and slightly higher activities of methionine adenosyltransferase (EC 2 -5.1 .6) and betainehomocysteine methyltransferase (EC 2.1.1.5; compared with other diets), suggesting that this diet encouraged re-methylation of homocysteine at the expense of trans-sulphnration to cystathionine. The findings obtained in these studies form a useful basis for further investigation of the metabolic interrelationships between methionine and related nutrients. The essential amino acid methionine fulfils a number of important functions in metabolism. Apart from being a constituent of proteins, it has a unique role as the initiating amino acid of all polypeptide synthesis, it donates methyl groups in a number of important synthetic reactions, it is a precursor of the polyamines spermine and spermidine and its sulphur moiety can be used in the synthesis of cysteine.The methylation role of methionine is fundamental to a number of areas of intermediary metabolism, e.g. amino acid metabolism, phospholipid synthesis, DNA and RNA synthesis and function, and is involved in numerous other pathways such as C, metabolism and creatine synthesis. Consequently methionine is known to interact with a number of other essential nutrients, the most obvious of these being cysteine (and sulphate), choline, vitamin B,, and folic acid. Interactions also exist with other amino acids such as arginine, glycine, serine and threonine (Keshavarz & Fuller, 1971 ;Girard-Globa et al. 1972).In order to investigate the nutritional interactions, information about the metabolism of the nutrients involved is required. The first step towards this is to measure how the activities of the enzymes involved change as the levels of the nutrients in the diet change. In the studies reported here, chicks were given one of five different diets varying in methionine or choline (or betaine) content for a period of 14 d. Growth, food intake and carcas...