This investigation examined ventilatory and blood lactate responses to maximal treadmill exercise during the early follicular (EF), late follicular (LF), and mid-luteal (ML) phases of the menstrual cycle. Subjects were moderately active women (n = 5), 20-24 years of age with regular menstrual cycles 25-36 days in length. Menstrual cycle phase (EF days 2-5; LF days 12-15; ML days 20-23) was verified by basal body temperatures and serum progesterone levels. Each subject performed a progressive incremental treadmill test to max during EF, LF, and ML. Metabolic and ventilatory variables and heart rate were measured continuously during the tests. Ventilatory Threshold (VT) was estimated from ventilatory parameters (VE-VO2 and VCO2-VO2 curves). Venous blood samples were withdrawn 5 min prior to exercise and 3 min post-exercise for hematocrit, lactate and progesterone analyses. Body weight and plasma volume changes were not different (p > 0.05) between the phases. Additionally, VO2 max, VE max, VCO2 max, post-exercise blood lactate and time to exhaustion were similar (p < 0.05) during the menstrual cycle. Relative VT occurred at a significantly higher (p = 0.02) percentage of VO2 max in EF compared to ML, and approached significance (p = 0.06) for EF compared to LF. We concluded that, with the exception of relative VT, metabolic and performance variables measured during maximal treadmill exercise were not dependent on menstrual cycle phase.