The effect of physical exercise on a "normal" and "high" protein intake(1 vs 2.5g/kg bw/24hrs) on plasma amino acid levels (PAAL) were followed during 24hr at energy balance in 8 and 6 healthy males, resp., after 6 days on a standardized diet and exercise program. The subjects were fasting until 12noon and then fed 10 identical meals hourly until 9PM. Physical exercise was performed at 45% of VO₂-max during fasting (8.30-10AM) as well as in a fed state (4-5.30PM). Energy turnover was determined by direct and indirect calorimetry and protein turnover evaluated by continuous infusion of ¹³C-leucine and ¹⁵N¹⁵N-urea. PAAL were determined every hr using an amino acid analyzer. Although the PAAL were increased during meals, a high protein intake resulted in lower PAAL values for most amino acids (AA), with exception of methionine, tyrosine and the branched-chain amino acids (BCAA). Physical activity slightly decreased BCAA levels at fasting but increased BCAA levels in fed state. Thus, a high protein intake at energy balance may not necessarily lead to increased availability of AA in peripheral tissues but PAAL appear to reflect the extent to which specific amino acids are involved in muscle and/or liver metabolism.