1. Six men were infused intravenously for 10 h with a tracer amount of l-[U-14C]tyrosine while on a standardized food intake.
2. Measurements of plasma l-[14C]-tyrosine specific radioactivity and the excretion rate of 14CO2 were made at frequent intervals and showed plateau labelling of plasma and expired carbon dioxide within 6–8 h. The tyrosine flux was calculated from the specific radioactivity in plasma at plateau value.
3. The excretion rate of 14CO2, corrected for retention of label within the bicarbonate pool, showed that oxidation accounted for 20% of the tyrosine flux. Urinary excretion of label was negligible.
4. Rates of protein synthesis were calculated from the flux of tyrosine after subtracting the proportion oxidized. Although the mean rate of synthesis was consistent with other measurements of protein turnover, the individual values ranged from 284 to 387 g/day. The variation was not reduced by relating turnover to body weight, lean body mass or energy expenditure.
5. Estimating the rates of protein breakdown from the tyrosine flux involved some assumptions about pathways of phenylalanine metabolism. The use of a labelled essential amino acid would therefore give more accurate values for short-term measurements of whole body protein turnover.