It is unclear whether duration of the interval separating CO2 rebreathing tests has any effect on the CO2 response curve. In normal subjects, we compared slopes and intercepts, respectively, of CO2 response curves from 3 rebreathing tests separated by each of three different time intervals: 10, 20, and 30 min. The slopes and intercepts, respectively were no different from the first through third test with each interval and no different between tests from different intervals. Therefore, reproducibility of slope and intercept was not affected by the time interval separating each test. During each rebreathing test, heart rate and systolic blood pressure increased but diastolic blood pressure decreased. These effects resolved within 10 min after the test. We conclude that, in normal subjects, 3 CO2 rebreathing tests can be repeated rapidly with only 10 min of rest separating each test.