We investigated the effects of the luteal phase of the menstrual cycle, as compared with the follicular phase, on ventilatory response (VR) and exercise performance in eight normally menstruating, non-athletic women. Subjects were studied near the predicted mid-point of each phase which was later documented by serum progesterone level. Resting VR to hypercapnia was greater, and VR to hypoxia tended to be greater in the luteal phase than in the follicular phase, but the increases in VRs were unrelated to progesterone level. There were no differences in maximal oxygen uptake, maximal duration of exercise, maximal heart rate, work efficiency, maximal ventilation (VE), anaerobic (ventilatory) threshold, gas exchange, cardiac output, or oxygen delivery. The PaCO2 was lower, and pHa tended to be higher during exercise. VE per unit CO2 output (VE/VCO2) was increased. R values (VCO2/VO2) were less, and maximal lactate values tended to be less, suggestive of increased dependence on fat for energy metabolism. At a given workload, exercise VE was unchanged due to the effect of less CO2 output at a given VO2 (lower R), balancing increased VE/VCO2. We conclude that, although ventilatory parameters are altered by the menstrual cycle, there is no overall effect on maximal exercise performance.