Our objective was to determine if the menstrual cycle affected expiratory resistance developed during progressive incremental exercise in females. Eleven females (age = 19.7 ± 1.1 yr., body mass = 58.9 ± 8.8 Kg, height = 1.65 ± 0.3 m) gave consent to participate in the study. Participants were studied during the follicular (day 7 ± 2 days following onset of menses) and luteal (day 21 ± 2 days following onset of menses) phases of their menstrual cycle. The expiratory resistance was significantly higher during the follicular phase at maximal workload versus the luteal phase (1.0 ± 0.06 cm H₂O/L/sec vs. 0.9 ± 0.07 cm H₂O/L/sec.: p¼ 0.05). No other differences were found in expiratory resistance, oxygen uptake or maximal heart rate during exercise. Results showed that the increase in expiratory resistance during the follicular phase of the menstrual cycle may be contributing to the changes in the pulmonary system of females as reported by other authors.