The maximal muscular power (both instantaneous, w, and average, w-.) and the cross-sectional area of the left thigh (CSA) were measured on six subjects before (B) and after (A) prolonged exposure to high altitude (above 5000 m asl). w and w were determined during a standing high jump off both feet on a force platform, and CSA by computed tomography. It was observed that: (1) in B, body weight (BW) = 74.1 +/- 5.8 kg, w = 3330 +/- 460 W (44.8 +/- 3.4 W.kg-1). w-. = 1795 +/- 395 W (24.6 +/- 4.3 W.kg-1), and CSA = 184.5 +/- 23.1 cm2; 2) in A, BW = 70.4 +/- 6.6 kg, w = 3005 +/- 472 W (42.5 +/- 3.6 W.kg-1), w = 1531 +/- 267 W (21.9 +/- 3.1 W.kg-1), and CSA = 163.5 +/- 23.1 cm2. Thus, w and w-. were decreased both in absolute terms (-9.8% and -14.7%, respectively) and per unit BW (-5.1% and -11.0%). However, because of the concomitant decrease in CSA, when expressed per unit cross-sectional area of the muscle, w (9.04 +/- 0.71 and 9.20 +/- 0.72 W +/- cm2) and w (4.87 +/- 0.81 and 4.70 +/- 0.67 W/cm2) were unchanged. The intrinsic capacity of the muscle to generate explosive power is therefore preserved in A. It is concluded that the decrease in w and w after high-altitude exposure depends only on a net loss of muscle mass.