=Effect of freezing rate on sperm survival.= Reports by one group of British workers in early trials on freezing bull semen indicated that the rate of cooling in freezing should not exceed 2 deg. C. per minute between +5 deg. and -15 deg. C., although below -15 deg. C. the rate could be faster. Another group expressed the view that semen could be plunged into dry ice at -79 deg. C. after it had been cooled to -15 deg. C. To clarify this part of the freezing procedure, 11 samples of semen were subdivided and portions of each were frozen at rates of 0.25 deg., 0.5 deg., 1.0 deg., 2.0 deg., and 4.0 deg. C. drop per minute between +5 deg. and -20 deg. C. and then twice these rates between -20 deg. and -79 deg. C. Vials of each ejaculate at +5 deg. C. were also plunged directly into an alcohol bath at -79 deg. C. The samples which were cooled at the rates of 0.25 deg., 0.5 deg., 1.0 deg., 2.0 deg., and 4.0 deg. C. per minute had the following percentages of motile sperm after thawing: 30, 40, 46, 44, and 44. A mean of 32 percent of the sperm in the samples that were plunged directly into an alcohol bath at -79 deg. C. were motile after thawing. There were no statistically significant differences among the samples frozen at 1.0 deg., 2.0 deg. or 4.0 deg. C. per minute. All of the others had significantly lower survival rates. Thus, it is obvious that too slow a cooling rate and plunging the samples directly into a -79 deg. C. bath from a temperature of +5 deg. C. cause greater harm to the sperm than cooling at a rate between 1.0 deg. and 4.0 deg. C. per minute. Some investigators have suggested that rapid cooling below -20 deg. C. is not detrimental to frozen semen. This idea was tested in conjunction with other experiments. Twenty-five samples cooled slowly (2 deg. C. per minute to -28 deg. C., then 4 deg. C. per minute to -79 deg. C.) showed 62 percent sperm survival compared with only 45 percent when cooled rapidly below -28 deg. C. (2 deg. C. per minute to -28 deg. C. then plunged into bath at -79 deg. C.). Thus, rapid cooling was detrimental even after the critical temperature range of +5 deg. C. to -20 deg. C. had been passed. [Illustration: Cooling rates of diluted semen samples in plastic vials and in glass ampules (Fig. 4)] =Rate of cooling in plastic and in glass.= Plastic vials do not conduct the cold as rapidly as glass ampules do. The temperature in both glass and plastic containers tends to lag behind the change in the bath in which they are immersed as is shown in Figure 4.