It is obvious that motility falls off rapidly after the semen is thawed. In a field trial in which the initial intent was to test the effect of glycerol levels on fertility of frozen semen, the semen was thawed in the morning and used during the same day. Survival of the sperm with 4 percent glycerol was so poor that only a few breedings were made with these samples. Even at 7 and 10 percent, the fertility results were much lower than with semen that had not been subjected to freezing. At that time it was felt that thawing the samples and using them throughout the day may have caused the low fertility results. Since then, a large-scale experiment by Cornell University investigators, in cooperation with the New York Artificial Breeders' Cooperative, has shown definitely that thawing should be delayed until a few minutes prior to breeding.[11] If the semen is used immediately, a thawing temperature of either 5 deg. or 38 deg. C. appears to be suitable. However, there is less danger of cold shock due to recooling if 5 deg. C is used. =Thawing rate in plastic and in glass.= Glass ampules transmit cold or heat more readily than plastic ones. The temperature rise is rapid in both glass and plastic when samples are taken from the storage box at -79 deg. C. and placed in water at 5 deg. C. However, complete thawing occurs more rapidly in glass than in plastic ampules. The changes in temperature that occurred when glass and plastic ampules were thawed in a water bath at 5 deg. C. are shown in Figure 8. The initial temperature rise for the first minute or two was about the same, then the rate of warming in the plastic slowed and actual melting of the frozen sample occurred a little over a minute later in the plastic than it did in the glass. Both were thawed in less than four minutes. [Illustration: Warming rates of diluted semen samples in plastic vials and in glass ampules (Fig. 8)] EFFECTS OF FREEZING PROCEDURES ON METABOLIC ACTIVITY OF BULL SPERM By finding how methods of handling affect the sperm cells, one can sometimes improve the procedures to avoid harmful effects. Some attempts have been made in this laboratory to determine the effects of the freezing procedures on the metabolic activity of bull spermatozoa. These investigations have been limited in scope, involving the measurement of oxygen-consumption and estimates of sperm motility during and after incubation at 37 deg. C. in a Warburg apparatus.