The testicles and their function

The testicles (or testes) are the male sex glands which produce sperm and testosterone, the male sex hormone. They are located outside of the body in the scrotum because sperm develop best at a temperature several degrees cooler than normal body temperature.

The testicles contain structures called seminiferous tubules which produce cells called germ cells. These in turn produce sperm. The sperm move into the epididymis (a tubular shaped structure located behind the testicle), where they are stored and then move into the body through a longer small tube called the vas deferens. With ejaculation sperm are mixed with fluids from other areas (the prostate and seminal vesicles) to form semen (ejaculate).

The testicles are initially formed inside the abdomen and gradually move into the scrotum before birth. In a small number of people this may not happen, and one testicle (or both) may remain in the abdomen. This is described as an undescended testicle. In some cases, the testicle may move into the scrotum. If this does not occur an operation(orchidopexy) may need to be performed. The usual timing of this is at around 1 year of age. 

It’s quite normal for one testicle to be slightly larger than the other and for one to hang a bit lower, although the size and shape should be roughly the same.


A round 90% of testosterone is produced by the testicles. Testosterone is the male sex hormone and is essential for the development of the reproductive organs and other male characteristics such as:

  • Body and facial hair
  • Low voice
  • Muscle development
  • The ability to have an erection.
  • Sex drive (libido)
  • Stamina
  • Mood and wellbeing

Without enough testosterone a man will lose his sex drive, suffer from fatigue, low mood and may gain weight. Testosterone is commonly associated with male aggression but, it is a hormone that helps men deal with the stress and strain of everyday life. Keeping physically fit and avoiding too much fatty fried food, sugar, alcohol and caffeine, all of which can reduce testosterone levels, can keep levels healthy.

Image explaining the role of testosterone in the male body.

Reviewed 1/2024

Testicular image by kind permission of the European Association of Urology.