Temperatures in the immersion bath were recorded in a 2-milliliter glass ampule containing 1 milliliter diluted semen and in an 8-milliliter plastic vial containing 2.5 milliliters of diluted semen. A second plastic vial and glass ampule filled to capacity with diluted semen showed a cooling rate almost identical to that shown in Figure 4. It was obvious from the comparison that samples in the plastic vials cooled slower than those in glass and that the volume of semen (at least the small volumes used) in the vials had little effect on the rate of cooling. In another experiment, it was shown that the volume of diluted semen in the ampule to be frozen (0.2, 1.0 or 5.0 ml.) had little or no effect on the survival of the sperm. STORAGE TEMPERATURE In freezing and storing bull sperm, an alcohol bath containing dry ice at a temperature of -79 deg. C. has been used as a cooling agent. In many areas, the availability of dry ice is limited and the cost is rather high. Mechanical means are available for obtaining temperatures as low as, or lower than, -79 deg. C. but for the most part they are expensive. If warmer temperatures were suitable for storing frozen semen, the ordinary deep-freeze, which operates at -15 deg. to -25 deg. C., might be used. =Storage at temperatures from -23 deg. to -79 deg. C.= In testing the effects of storage temperatures on the survival of frozen bull sperm (in a diluent containing 7 percent glycerol), 9 ejaculates were frozen and kept at -23 deg., -37 deg., -51 deg., -65 deg., and -79 deg. C. The desired temperatures were maintained by dropping pieces of dry ice into ethyl alcohol baths as needed. Samples were thawed after 1 hour, 1 day, 3 days, and 5 days. After 1 hour, the samples maintained at the various temperatures exhibited approximately equal motility (Fig. 5). [Illustration: Effect of freezing and storing bull sperm at various temperatures on the sperm motility at thawing (average of 9 ejaculates) (Fig. 5)] At the end of 1 day, samples stored at -79 deg. C. exhibited approximately the same motility as did similar samples stored for 1 hour. The samples stored at -65 deg. C. had declined slightly in motility and those maintained at -51 deg. C. had only one-third the motility which they had displayed at 1 hour. The samples at -23 deg. and -37 deg. C. exhibited practically no motility after 1 day in storage. After 5 days, only 3 of the 8 ejaculates stored at -51 deg. C. showed motility upon thawing. Apparently detrimental changes take place more rapidly when the samples are stored at temperatures warmer than -65 deg. C. The nature of these changes has not been determined. Reports from other laboratories indicate that storage temperatures much lower than -79 deg. C. are just as satisfactory as -79 deg. C.