1. Five healthy male subjects were studied by continuous infusion of l-[α-15N]lysine over 20–30 h with timed blood and urine samples, and two or three percutaneous needle biopsies of vastus lateralis muscle.

2. A standard creatine-free diet, quantitatively related to body surface area, was given for 5 days before the infusion. The [15N]lysine was administered at a constant rate in an amino acid solution with a nitrogen content of 0·96 mol/l, which constituted the sole source of exogenous nitrogen during the infusion.

3. A plateau level of plasma free [15N]lysine enrichment was achieved after infusion for 14 h. The total plasma lysine flux calculated from the plateau was 7·3 mmol/h (range 4·8–9·6). Total body protein turnover calculated from the lysine flux was 3·5 g day−1 kg body wt.−1 (range 2·5–5·0).

4. Muscle sarcoplasmic and myofibrillar fractions were separated, purified and the 15N enrichment was measured. The sarcoplasmic protein fractional synthetic rate was calculated as 3·8%/day (range 2·2–5·1). The myofibrillar protein synthetic rate was 1·46%/day (range 1·09–2·44).

5. Muscle mass, calculated from 24 h creatinine excretion, was 33·7 kg (range 28·8–37·4), which represented 500% of body weight (range 38·9–58·1). Total muscle protein synthesis was calculated to account for 53·2% (range 39·5–62·1) of total body protein synthesis.

6. The advantages and limitations of using continuous infusion of [15N]lysine in human subjects are discussed.

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