GLYCEROL ADDITIONS 17 Effect of glycerol on sperm survival at 5 deg. C. 17 Glycerol levels for freezing semen 18 Rate, temperature, and method of adding glycerol 18 Allowing sperm to equilibrate with glycerol 20 Sugar additions and equilibration time 20 Substitutes for glycerol 22 FREEZING RATE 22 Effect of freezing rate on sperm survival 22 Rate of cooling in plastic and in glass 23 STORAGE TEMPERATURE 24 Storage at -23 deg. to -79 deg. C. 24 Use of higher glycerol levels and storage at -20 deg. C. 25 THAWING 26 Comparison of thawing at 5 deg. C. and at 38 deg. C. 27 Thawing rate in plastic and in glass 29 EFFECTS OF FREEZING PROCEDURES ON METABOLIC ACTIVITY OF SPERM 30 Effect of glycerol additions on oxygen uptake of diluted semen at 37 deg. C. 30 Effect of glycerol-plus-catalase on oxygen uptake of diluted semen 32 Effect of freezing procedures on oxygen utilization 33 Effect of freezing procedures on methylene-blue reduction time 33 PRACTICAL FREEZING PROCEDURE 35 Collection of semen 35 Preparation of extender 35 Dilution after collection 36 Adding the glycerol 36 Equilibration 36 Freezing 37 Thawing 37 LITERATURE CITED 38 TEMPERATURE CONVERSION TABLE 39 Urbana, Illinois October, 1957 Publications in the Bulletin series report the results of investigations made or sponsored by the Experiment Station ACKNOWLEDGMENT The authors gratefully acknowledge the financial assistance given to the Department of Dairy Science in support of these investigations by the Southern Illinois Breeding Association of Breese, Illinois, and the Northern Illinois Breeding Co-op of Hampshire, Illinois, through the Illinois Dairy Breeding Federation. THE PRESERVATION OF BULL SEMEN AT SUB-ZERO TEMPERATURES By N. L. VANDEMARK, W. J. MILLER, W. C. KINNEY, JR., CARLOS RODRIGUEZ, and M. E. FRIEDMAN[A]