Abstract
In dogs and men performing exercises of varying severity, O2-debt was found to be quantitatively unrelated to blood lactate concentration, and to change with time in an entirely different manner. The errors of lactate were both positive and negative, tending toward the latter with physical training. When the error was positive blood pyruvate was high, but pyruvate concentrations were low when lactate was excessively low. A function, "XL", calculated from both lactate and pyruvate concentrations was closely related quantitatively to O2-debt under these circumstances. Its oxygen equivalence was 0.5 M O2/M and its virtual volume of distribution was the volume of body water.