We have previously found that total 24h leucine (leu) and nitrogen (N) balances were neutral when 180 min of exercise (45-50% VO₂max) were performed daily (90 min in fasting and 90 min in feeding) for 7 consecutive days, under conditions of neutral energy balance and a dietary protein intake of Ig· kg^-1· d^-1. We have now assessed whether an increase in protein intake would induce a retention of leu and/or N. Four healthy men were studied on day 7, of a diet supplying 2.5 g· kg^-1·d2, by using primed constant intravenous infusions of [1-¹³C]leu and [¹⁵N¹⁵N]urea, in combination with both indirect and direct (suit) calorimetry. 24h leu balance (input-oxidation) was+16.1±8.4mg- kg^-1·d^-1 (MeaniSD) for an intake of 277 mg leu· kg^-1·d^-1. Hence, leu balance was positive by about 5.8% of intake. 180 min exercise activity induced a total increase in daily leucine oxidation by 4.1%. 24h N excretion (corrected for urea N pool size) was 366±29 mg N· kg^-1 · d^-1, for an intake of 391 mg N·kg^-1·d^-1. These results indicate a mild positive retention of leu (p<0.01) and N (p<0,01)under these conditions. It is possible that a more beneficial effect of an increase in protein intake would more likely be found when energy balance is negative. NIH grants RR88, DK15856, SJFR 50.0204/94, CIF 55194.