Study population: Fifty healthy menstruating women will be included in the study and randomized to treatment with testosterone or placebo. Inclusion criteria: 18-35 yrs of age; body mass index (BMI) 19-25; non-smoking; a moderate to high self-reported level of recreational physical activity; not taking hormonal contraception and willing to use highly efficient non-hormonal contraception during the study (intrauterine device, bilateral tubal occlusion, vasectomised partner, same-sex partner, or sexual abstinence); accepting to not participate in any sports competitive event during the study period plus one month. Exclusion criteria: the presence of cardiovascular, liver, biliary or renal disease; hyperlipidemia; uncontrolled high blood pressure; endocrinological disorder; oligomenorrhea (menstrual intervals of more than 6 weeks) or amenorrhea (no menstruation for at least 3 months); pregnancy; a history of thromboembolic disorder; any malignancy; and intake of hormonal contraception the last two months prior to the study.