The male reproductive system depends upon the action of many different hormones or chemicals, produced by various body glands and enter systemic circulation. Some of these hormones, called "tropic" hormones, cause other hormones to release. Other hormones have direct effects upon organs or body systems, emotions and production of semen. Unlike women, men don't experience cyclic hormone fluctuation throughout the month--instead, their hormone levels stay relatively constant throughout their reproductive years.
Something of a "master" hormone, according to the textbook "Human Physiology," Gonadotropin-Releasing Hormone (GnRH) is a tropic hormone produced by a part of the brain called the hypothalamus. While GnRH isn't directly responsible for male sexual behavior or characteristics, it nevertheless proves incredibly important, because it causes the release of two other hormones of the male reproductive system.
Produced in a part of the brain called the anterior pituitary, follicle-stimulating hormone (FSH) proves active in both male and female reproductive systems. The name comes from the hormone's action in females—males don't produce follicles—but the same hormone responsible for development of a mature egg in women stimulates the production of sperm in the testes of men. FSH is released in response to the stimulation of the anterior pituitary by GnRH.
Like FSH, luteinizing hormone (LH) is released by the anterior pituitary in response to the action of GnRH. Also like FSH, LH is produced by women as well and named for its action in the female reproductive cycle—men don't experience luteinization, which is the release of a mature egg from the ovary during ovulation. In men, LH causes the interstitial cells of the testes to produce the hormone testosterone.
Made in the testes, testosterone enters systemic circulation in relatively constant concentrations in a healthy, reproductive-age male. This hormone produces and maintains the secondary sexual characteristics of the male—enhanced musculature, facial and body hair, thickened larynx and deepened voice and enlargement of the genitals. It's also responsible for the sex drive and works with FSH to stimulate the production of sperm. The Cleveland Clinic notes that while low testosterone levels in aging men contribute to loss of sex drive and some secondary sexual characteristics, hormone replacement leads to increased risk of prostate cancer and atherosclerosis.
The hormone inhibin is produced by cells in the testes that are responsible for monitoring the health and maturation of sperm. If sperm levels are high, making nutrients for the developing sperm scarce, the testes release inhibin. The inhibin travels through the bloodstream to the brain, where it prevents the secretion of GnRH. In the absence of GnRH, FSH and LH levels fall and sperm production slows. This is one of the major mechanisms whereby male hormones are maintained at relatively constant concentration.