The release of sperms from an antheridium of Polytrichum juniperinum occurs in two phases: a rapid phase, completed within a few seconds, and a slow phase, usually completed in 1–5 min. The rapid phase depends on an elastic contraction of the jacket that forces all or most of the sperms out of the chamber. The contraction has this effect because the base of the chamber contains a fluid that accounts for more than half the volume of the chamber. The fluid accumulates during maturation of the antheridium, and the jacket is stretched, storing the potential energy required for the expulsion of the sperms. The rapid phase of sperm release occurs in hypertonic solutions and in petroleum jelly, which confirms that the uptake of water by the antheridium is not required for the opening of the antheridium. The slow phase of release is distinguishable only when a substantial residue of sperms remains after the rapid release, and can be prevented by high osmotic concentration in the bathing medium. The slow release probably depends on the net uptake of water by osmosis, resulting from the relatively high osmotic concentration of the fluid in the antheridial chamber.