Respiratory gas exchange was measured during maximal treadmill exercise testing in six healthy volunteers and 20 patients with chronic heart failure. A curve of equation y = ax-bx2 was used to model the relation between the rate of oxygen consumption (y axis) and the rate of carbon dioxide production (x axis). The constants "a" and "b" were used to calculate the maximal value of the expression ax-bx2. This value was termed the "extrapolated maximal oxygen consumption". For all subjects a close fit between experimental data and mathematical model was obtained and the values of the measured maximal rate of oxygen consumption and "extrapolated maximal oxygen consumption" were similar. Respiratory gas exchange was reanalysed using only those values obtained during the first 90%, 75%, and 66% of exercise. In contrast with the value for the measured rate of oxygen consumption, the value of "extrapolated maximal oxygen consumption" was effectively independent of exercise duration. Extrapolated maximal oxygen consumption provides an objective measure of cardiorespiratory functional reserve that, within limits, is independent of exercise duration. Extrapolated maximal oxygen consumption is complementary to the direct measurement of the maximal rate of oxygen consumption and increases the amount of information derived from a single exercise test.