No previous exercise studies in women have assessed the effects of the normal menstrual cycle on the lactate threshold (LT) measured during a graded, maximal exercise test. This is relevant to our understanding of exercise training and metabolism in eumenorrheic women. The present study, therefore, examined the effect of menstrual cycle phase on the LT. Eight moderately active, eumenorrheic women performed three maximal exercise tests with simultaneous determination of LT. Tests were performed in the early follicular (low estrogen and progesterone), midfollicular (elevated estrogen and low progesterone), and midluteal (elevated estrogen and progesterone) phases of the menstrual cycle. No significant differences were observed in LT measured across phases of the menstrual cycle whether data were expressed in absolute terms (1,299 ± 70, 1,364 ± 80, and 1,382 ± 71 ml O2/min, respectively) or relative to maximal oxygen uptake (V̇o2 max; 52.1 ± 1.7, 54.7 ± 1.7, and 55.7 ± 1.6%, respectively). In addition, there were no significant cycle phase differences in V̇o2 max, maximal heart rate, heart rate at LT, or final lactate concentration. With data combined across all phases of the menstrual cycle, there was a significant correlation between the LT and the epinephrine breakpoint ( r = 0.91, P < 0.0002) and norepinephrine breakpoint ( r = 0.94, P < 0.0001). For epinephrine only, there was close correspondence between the epinephrine breakpoint (ml O2/min) and the LT. In conclusion, LT as well as V̇o2 max and other measures of cardiorespiratory fitness are not significantly affected by the changing sex steroid levels observed across the normal menstrual cycle. Data suggest that the onset of the steep increase in epinephrine determines the LT during graded exercise.