The most widely accepted theory to explain ejaculation consists of two phases: 1) A phase of accumulation of the various constituents of semen inside the prostatic urethra. 2) A phase of expulsion with opening of the striated sphincter, while the smooth sphincter of the bladder neck remains closed. The objective of this study was therefore to confirm or invalidate this hypothesis.Eight volunteers were studied by continuous transrectal ultrasonography allowing dynamic analysis of ejaculation. Sixteen patients presenting with a bladder neck obstruction syndrome were treated by a technique preserving the last centimetre of supramontanal urethra.In the eight healthy subjects, we constantly and successively observed: filling of the ejaculatory ducts. prostatic contractions. Expulsion of the contents of the seminal vesicles into the inframontanal urethra, without prior ballooning of the prostatic urethra. In the 16 patients treated for bladder neck obstruction, we did not observe any case of retrograde ejaculation.