A valve assembly adapted for implantation into the vas deferens of a male patient. The valve assembly is operable in a first mode to block the flow of sperm and in a second mode to permit the flow of sperm. The surgeon is permitted to shift the valve assembly between its first and second functional modes by moving a moveable valve member. The valve assembly includes a valve retention feature for releaseably retaining the valve member is a selected position associated with each of the two distinct functional modes.
