The ability of the human to perform physical work, his tolerance to it and those factors that limit the amount he can do, have been the subject of much investigation. As a result of earlier work most investigators have become interested in maximum values of respiratory and heart rate parameters, i.e., maximum VO2 1/min and maximum heart rate. Attempts to show correlation between these maximum values and the amount of work done have been made with varying degrees of success. However, little attention has been paid to the way in which maximum values have been reached. The suggestion that one parameter may be the limiting factor, in work capacity, with healthy humans has been made frequently, An attempt has been made to examine the relationship between the physiological parameters and work capacity using a highly skilled group of subjects, habituated to the task. The work task was constructed so that there would be validity between it and the subjects' athletic ability, therefore giving high specificity. A cycle ergometer was used and the subjects, n = 12, were time trial cyclists skilled in riding at constant speeds against the clock.