The prostate is part of the male reproductive system. It is about the size of a walnut and weighs about an ounce. The prostate is located below the bladder and in front of the rectum. The prostate goes all the way around a tube called the urethra, which carries urine from the bladder out through the penis. The main job of the prostate is to make fluid for semen. During ejaculation, sperm made in the testicles moves to the urethra. At the same time, fluid from the prostate and seminal vesicles also moves into the urethra. This mixture – semen- goes through the urethra and out of the penis.