Tostes RC, Carneiro FS, Carvalho MH, Reckelhoff JF. Reactive oxygen species: players in the cardiovascular effects of testosterone. Am J Physiol Regul Integr Comp Physiol 310: R1-R14, 2016. First published November 4, 2015 doi:10.1152/ajpregu.00392.2014.-Androgens are essential for the development and maintenance of male reproductive tissues and sexual function and for overall health and well being. Testosterone, the predominant and most important androgen, not only affects the male reproductive system, but also influences the activity of many other organs. In the cardiovascular system, the actions of testosterone are still controversial, its effects ranging from protective to deleterious. While early studies showed that testosterone replacement therapy exerted beneficial effects on cardiovascular disease, some recent safety studies point to a positive association between endogenous and supraphysiological levels of androgens/testosterone and cardiovascular disease risk. Among the possible mechanisms involved in the actions of testosterone on the cardiovascular system, indirect actions (changes in the lipid profile, insulin sensitivity, and hemostatic mechanisms, modulation of the sympathetic nervous system and renin-angiotensin-aldosterone system), as well as direct actions (modulatory effects on proinflammatory enzymes, on the generation of reactive oxygen species, nitric oxide bioavailability, and on vasoconstrictor signaling pathways) have been reported. This mini-review focuses on evidence indicating that testosterone has prooxidative actions that may contribute to its deleterious actions in the cardiovascular system. The controversial effects of testosterone on ROS generation and oxidant status, both prooxidant and antioxidant, in the cardiovascular system and in cells and tissues of other systems are reviewed. cardiovascular; prooxidant; antioxidant TESTOSTERONE 1 IS THE PREDOMINANT and most important androgen, playing a major role in the development of male reproductive tissues (130,135). "Androgen" is a broad term for any natural or synthetic compound that primarily influences the growth and development of the male reproductive system, including the activity of the accessory male sex organs and development of male secondary sex characteristics (21, 135). Androgens are also essential for health and well being, and their beneficial and stimulant effects have been known for a long time, since the seminal studies from Brown-Séquard in the nineteenth century 2 (22, 135): "ь ь ь in the seminal fluid, as secreted by the testicles, a substance or several substances exist which, entering the blood by resorption, have a most essential use in giving strength to the nervous system and to other parts ь ь ь ."Other androgens include androstenedione, 5␣-dihydrotestosterone (DHT), which is produced from testosterone by the enzyme 5␣-reductase, dehydroepiandrosterone (DHEA), DHEA sulfate (DHEA-S), and synthetic androgens in their many steroid ester variations (e.g., testosterone cypionate, decanoate, undecanoate, enant...