1282 Although Mabel Fitzgerald showed as early as 1913 that women do not respond to altitude as men do, their response has been understudied until now. Results of a multi-year, multi-disciplinary collaborative study of acclimatization to 4300 m in women will be presented. The hypotheses of the investigations included that women would acclimatize more rapidly during the luteal phase of the menstrual cycle than during the follicular, and that women would acclimatize more easily than men. Data to be presented include: 1) evaluation of the energy requirements for weight maintenance, 2) acute and chronic respiratory responses (hypercapnic and hypoxic ventilatory drive), 3) cardiovascular responses (blood pressure, heart rate and cardiac output), 4) evaluation of regulation of blood volume (volume regulatory hormones and plasma volume), and 5) fuel substrates used at rest and during moderate exercise. In all cases, comparisons will be made between responses during the follicular and luteal phases of the menstrual cycle, between sea level and altitude responses, and where possible, with responses in men studied under similar circumstances.