Before freezing, embryos are first equilibrated in special solutions containing cryoprotecting agents. These agents protect the embryos from intracellular ice formation, which would be detrimental to their viability. The embryos are then placed in special straws, sealed and put in a machine with an integrated computer. This machine, the programmable freezer, lowers the temperature in a slow controlled manner until -196oC. There are several protocols of slow freezing, depending on the stage of the embryos and the type of cryoprotectant solutions used. The embryos are then plunged in liquid nitrogen and are stored until used.

Usually 1-3 embryos are placed in each straw. In this state, embryos may be preserved for a very long period of time. Embryos can be frozen at the 2PN stage (Day 1), cleavage stage (2-8 cells; Days 2-3), or blastocyst stage (Days 5-6 post oocyte retrieval).

Cryopreservation in liquid nitrogen at a temperature of -196oC does not require electric power. The only requirement is the replenishment of liquid nitrogen in the tanks containing the embryos.