Four women, aged 20-35 years, exercised at approximately 30% VO2 max in three environments (28 degrees C, 12.6 torr vp; 35 degrees C, 28.0 torr vp; 48 degrees C, 8.7 torr vp) during three stages at the menstrual cycle: ovulation, luteal phase, and flow. Cycle phase was confirmed by hormone analysis for estradiol-17B and progesterone in venous blood samples. Although pre-exposure care temperature and oxygen uptake were higher in the luteal phase, these differences disappeared under the combined influence of exercise and heat stress. Exercise heart rate, care temperature, and sweat rate were similar in all cycle phases. Oxygen uptake, ventilatory volume, cardiac output, stroke volume, blood electrolytes, blood pressure, body weight, and tolerance time were unaffected by cycle phase. In the 28 degrees C environment, mean skin temperature and forearm blood flow were lower in the luteal phase than during flow, but this difference was not observed in 35 degrees C or 48 degrees C. A greater relative decrease in plasma volume was observed in the luteal phase following exercise in 48 degrees C than during flow. However, the overall response of women to exercise and heat stress during three phases of the menstrual cycle suggest that minor cyclic alterations in physiological systems apparent at rest are masked by the demands of the activity and the environment.