A group of presumably healthy, sedentary men participated in a physical fitness program for seven months. Their maximum oxygen intake was determined during either a standard treadmill test or an "all-out run" or both. Eighteen men tested on the treadmill before and after the test had an increase in their peak oxygen uptake from 31.3 to 36.8 ml/kg/min (P<0.001). Ten of the subjects who were tested by both methods before and after seven months of training had statistically significant increases in their peak oxygen intakes. Their peak performance was consistently higher with the "all-out run" than it was on the treadmill. Eight men failed to increase their peak energy expenditure following training; four of these subjects were tested twice with the treadmill test and their peak performances remained the same before and after the program.