The O2 equivalent of lactic acid accumulation in blood was estimated from data on ten subjects acclimatized to altitudes up to 6500 m in the course of a Mt. Everest expedition. The calculated value of the O2 equivalent of La by standard least squares regression was 2.07 +/- 0.4 l O2 for a rise of blood lactic acid concentration [Lab] of 1 g per liter of blood (2.92 ml O2 per kg of body weight per 1 mM increase in blood lactate). This value, as well as the value obtained by the regression procedure suggested by Brace (2.84 +/- 0.5 l), was not significantly different from the previously reported value of 2.38 +/- 0.22 (14) for sea-level conditions. This finding indicated that, after La equilibrium is achieved, the redistribution of La among extra- and intracellular body compartments, as reflected by [Lab], appears not to be substantially affected by chronic hypoxia. The calculated maximum lactic O2 debt values at the Mt. Everest base camp (5350 m) were 1.26 and 1.42 liters for acclimatized Caucasians and for Sherpas, respectively. The amount of the measured maximum O2 debt at altitude was somewhat increased by pure O2 breathing (1.53 vs 1.26 l).