If the delay time between gas concentration and flow signals is not adequately corrected during breath-by-breath analysis of expired gas, an error in calculation of oxygen consumption (Vo2) will result. To examine the frequency and delay time dependences of errors in Vo2 measurement, six healthy men exercised at 100, 200, and 250 W on a cycle ergometer while breath-by-breath assessment of Vo2 was made simultaneously with collection of expired air. Subjects breathed first at normal rates (15-30 breaths/min) and then at 70 breaths/min. Each subject performed each level of exercise twice by using erroneous values for the delay time between gas concentration and flow signals. At normal breathing frequencies, errors in Vo2 measurement were +/- 10% over the full range of delay times used, and the errors were not tightly correlated with variations in delay times from optimum. However, at 70 breaths/min, errors approached +/- 30% as the variations in delay times deviated +/- 0.1 s from the optimal, and the errors were highly correlated with the variations in delay times. We conclude that there is greater potential for errors in Vo2 measurement with incorrect delay time at higher breathing frequencies. These findings suggest that the optimal delay time for breath-by-breath systems should be adjusted by using high breathing frequencies.