Annually, about a half million men in this country who want to practice reliable birth control without placing the burden on their partners undergo vasectomies. This minimally invasive procedure involves cutting and tying the ends of the tubes that deliver sperm from the testes (vas deferens). This prevents sperm from mixing with the seminal fluid. Thus, at ejaculation, the semen contains no sperm. Although the testes will continue to produce sperm, they no longer pass through the vas deferens. Instead, they die and are absorbed by the body. Because semen contains about 95% seminal fluid, there is virtually no discernible difference in the ejaculate. Since the testes continue to produce testosterone, there is no effect upon a man's sex drive.