This research will determine if: 1) Oral progesterone attenuates drug-induced QT interval, J-Tpeak and Tpeak-Tend lengthening in postmenopausal women 50 years of age or older, and 2) Transdermal testosterone attenuates drug-induced QT interval, J-Tpeak and Tpeak-Tend lengthening in men 65 years of age or older. This investigation will consist of two concurrent prospective, randomized, double-blind, placebo-controlled crossover-design studies in a) Postmenopausal women, and b) Men 65 years of age or older. Study 1: Each postmenopausal woman will take progesterone or placebo capsules for 1 week. After a 14-day 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. Study 2: Each man 65 years of age or older will apply transdermal testosterone or transdermal placebo gel for 3 days. After a 7-day washout (no testosterone or placebo) each subject will then apply the alternative therapy (testosterone or placebo gel) for 1 week. After 3 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 testosterone and placebo phases will be compared.