Experiments in this and other countries on the preservation of spermatozoa at very low temperatures have shown that no mammalian spermatozoa so far examined survive freezing when they are cooled ultra-rapidly from temperatures above freezing point to temperatures of -79 degrees C or below. Slow cooling and the addition of glycerol to the media in which the spermatozoa are suspended, however, permits survival of the spermatozoa of many species. In different animals, there are marked variations in the resistance of their spermatozoa to freezing and the proportion of spermatozoa which can be revived from very low temperatures may be influenced both by the concentration of glycerol added to the semen and by the composition of the diluting fluid. In experiments with the spermatozoa of the bull, ram, stallion and boar it has been found that during slow cooling to -79 degrees C there is a critical temperature range between -15 and -25 degrees C at which the greatest amount of damage occurs. The rate at which the capacity for motility of the spermatozoa is destroyed within this critical temperature range is considerably reduced by allowing the spermatozoa to stand at 2 degrees C in contact with a medium containing egg yolk and glycerol for 18 h before freezing. The extent of damage in the critical temperature range may also be reduced by cooling the specimens at a rate of 0.25 to 0.5 degrees per second between -15 and -25 degrees C.