Skeletal muscle is a tissue with the primary function of enabling body movement. However, muscle also comprises most of the lean body mass in adult man (Forbes, 1987) and in addition to its contractile function it is involved in day-to-day regulation of whole-body amino acid and carbohydrate metabolism (Felig, 1975) as well as in adaptations to starvation (Cahill, 1970), disease and injury over a much longer term (Rennie, 1985b). Muscle in adults is commonly agreed to account for about 40% of the body weight, having grown from about 25% in the newborn (Cheek, 1968; Forbes, 1987). The amount of protein comprising the adult musculature means that it represents a great metabolic investment and one which is therefore relatively costly to acquire and maintain (Reeds et al, 1985). Muscle mass as a proportion of body weight is lower after puberty in women than men (Cheek, 1968; Forbes, 1987) and muscle mass and composition are not constant with ageing; there is a progressive fall in the total muscle mass (Forbes, 1987) and a selective loss of type II muscle fibres (Campbell et al, 1973; Aniansson et al, 1978; Larsson, 1978). The metabolic activities of muscle include both storage of protein (only one-sixth of which is dispensable without irreversible loss of muscle function) and of carbohydrate (much more limited because muscle glycogen accounts for only 1.5% of the wet weight; Saltin et al, 1977) and metabolic interchange. For protein, the latter function is confined to protein synthesis and breakdown and to a few steps of amino acid metabolism, e.g. transamination of the branched-chain amino acids (Krebs, 1972) and alanine; activities of glutamate and the branched-chain ketoacid dehydrogenases (Khatra et al, 1977); the involvement of aspartate in the purine nucleotide cycle (Lowenstein and Goodman, 1978); and glutamine synthesis (Smith et al, 1984). Glutaminase activity is mainly mitochondrial and thus ought to exist in muscle mitochondria, but the total tissue activity is low. Muscle's capacity for intermediary amino acid metabolism is demonstrated by the facts that limb export of glutamine and alanine is much greater than expected and that the output of branched-chain amino acids, from which they are synthesized,