Mesterolone (Proviron) is a derivative of the hormone testosterone. Testosterone is a naturally occurring male hormone known as an androgen. It is produced by the testicles and is the main hormone essential for normal growth and development of the male sex organs and male sexual characteristics. During adult life, testosterone is essential for the production of sperm, the maintenance of sex drive, erectile potency, and the functioning of the prostate gland and other reproductive structures. The natural production of testosterone is controlled by another set of hormones called gonadotrophins, which are released from the pituitary gland in the brain. Low levels of testosterone can cause impotence, decreased sex drive, decreased mental and physical activity, infertility due to decreased sperm production and bone loss. Mesterolone is given when natural testosterone levels are too low. This occurs following castration, or may be due to decreased functioning of the testicles (hypogonadism or eunuchoidism). This can be causedby testicular disease or a deficiency in gonadotrophin production by the pituitary gland. Mesterolone therefore allows the return of normal sexual functioning, and is useful in the treatment of infertility caused by low testosterone levels.