Testosterone is a hormone that plays an important role in the development and maintenance of male characteristics. It is produced mainly by the testes, but also to some extent by the adrenal glands and even the brain. Testosterone is responsible for the growth of facial hair, deepening of the voice, increase in muscle mass, and the production of sperm. In addition, it contributes to overall energy levels, mood, and libido (sex drive). Levels of testosterone fluctuate throughout a man's life, with peak levels occurring during adolescence and early adulthood, followed by a gradual decline as he ages.