An improved method of storing fowl semen in liquid nitrogen is described. It uses glycerol as a cryopotectant and the degree of fertility obtained with semen stored for up to 2 1/4 years was very promising. 2. The essential features of the method are a slow freezing rate, a reduction of temperature to - 35 degrees C before the semen is plunged into liquid nitrogen and insemination within 15 min of thawing and removing the glycerol from the diluent. 3. It seems likely that further improvements in the technique may be expected through better control of the injection of liquid nitrogen into the freezing chamber, thereby minimising localised, rapid temperature changes around the ampoules during freezing. 4. Preliminary results suggest that it may prove possible to select males for the ability of their spermatozoa to withstand freezing. 5. The importance of recognising differences between males in the initial quality of semen and between females in the physiology of the reproductive organs in determining the fertilising ability of stored semen is discussed.
