The purpose of the present study was to investigate the effect of exercise induced hyperventilation and hypocapnia on airway resistance (Raw), and to try to answer the question whether a reduction of Raw is a mechanism contributing to the increase of endurance time associated with a reduction of exercise induced hyperventilation as for example has been observed after respiratory training. Eight healthy volunteers of both sexes participated in the study. Cycling endurance tests (CET) at 223 (SD 47) W, i.e. at 74 (SD 5)% of the subject's peak exercise intensity, breathing endurance tests and body plethysmograph measurements of pre- and postexercise Raw were carried out before and after a 4-week period of respiratory training. In one of the two CET before the respiratory training CO2 was added to the inspired air to keep its end-tidal concentration at 5.4% to avoid hyperventilatory hypocapnia (CO2-test); the other test was the control. The pre-exercise values of specific expiratory Raw were 8.1 (SD 2.8), 6.8 (SD 2.6) and 8.0 (SD 2.1) cm H2O.s and the postexercise values were 8.5 (SD 2.6), 7.4 (SD 1.9) and 8.0 (SD 2.7) cm H2O.s for control CET, CO2-CET and CET after respiratory training, respectively, all differences between these tests being nonsignificant. The respiratory training significantly increased the respiratory endurance time during breathing of 70% of maximal voluntary ventilation from 5.8 (SD 2.9) min to 26.7 (SD 12.5) min. Mean values of the cycling endurance time (tcend) were 22.7 (SD 6.5) min in the control, 19.4 (SD 5.4) min in the CO2-test and 18.4 (SD 6.0) min after respiratory training. Mean values of ventilation (VE) during the last 3 min of CET were 123 (SD 35.8) l.min-1 in the control, 133.5 (SD 35.1) l.min-1 in the CO2-test and 130.9 (SD 29.1) l.min-1 after respiratory training. In fact, six subjects ventilated more and cycled for a shorter time, whereas two subjects ventilated less and cycled for a longer time after the respiratory training than in the control CET. In general, the subjects cycled longer the lower the VE, if all three CET are compared. It is concluded that Raw measured immediately after exercise is independent of exercise-induced hyperventilation and hypocapnia and is probably not involved in limiting tcend, and that tcend at a given exercise intensity is shorter when VE is higher, no matter whether the higher VE occurs before or after respiratory training or after CO2 inhalation.