The effects of two levels of protein intake on muscle performance and energy metabolism were studied in humans submitted to repeated daily sessions of prolonged exercise at moderate altitude. For this purpose, 29 healthy males, were exposed to seven successive stages of ski-mountaineering at altitudes between 2500 and 3800 m, and to an isocaloric diet (4000 kcal.day-1, 16,760 kJ.day-1) with either 1.5 g.kg-1.day-1 (C group, n = 14), or 2.5 g.kg-1.day-1 (PR group, n = 15) protein intake. Measurements made after the ski-mountaineering programme did not show any change in body mass. The peak torque during maximal isometric voluntary contraction (MVC) of the quadriceps muscle was unaffected by the repeated exercises, whereas the endurance time at 50% MVC was decreased in PR subjects (-26.8%, P < 0.001). Increased levels of both free fatty acids (+ 147%, P < 0.001) and glycerol (+ 170%, P < 0.001) observed in C subjects would suggest that lipolysis was enhanced after the repeated exercise. The plasma amino acid pattern was altered after completion of the ski-mountaineering programme; the plasma concentration of the three branched-chain amino acids (BCAA) was significantly decreased in C subjects, whereas the higher level of protein intake (PR group) greatly minimized the exercise-induced decrease in serum BCAA.