This research will determine if oral progesterone attenuates drug-induced QT interval lengthening in a) Postmenopausal women 50 years of age or older, and b) Premenopausal women studied during the ovulation phase of the menstrual cycle. This investigation will consist of two concurrent prospective, randomized, double-blind, placebo-controlled crossover-design studies in a) Postmenopausal women, and b) Premenopausal women. Each subject will take progesterone or placebo capsules for 1 week. After a two-week washout (no progesterone or placebo) each subject will then take the alternative therapy (progesterone or placebo) for 1 week. After 7 days of each treatment, subjects will present to the clinical research center to receive a small dose of the QT interval-lengthening drug ibutilide, and the effect on the QT, J-Tpeak and Tpeak-Tend intervals during the progesterone and placebo phases will be compared