No tests of the effects of storage at -79 deg. C. for periods longer than 51 days have been conducted in this laboratory. Portions of 12 ejaculates were frozen and stored at -79 deg. C. for various periods. One portion of each of these was examined on the second, ninth, 16th and 51st day of storage. The percent of motile sperm and rate of motility at each of these examinations were as follows: Day 2 9 16 51 Percent of motile sperm 49 46 40 38 Rate of motility 2.5 2.3 2.2 2.2 The average prefreezing motility percentage for the above samples was 58, with an average rate of motility of 2.9. It is apparent from these results that the loss in motility was greatest due to the initial freezing, and after that the drop was most pronounced during the first 16 days of storage. The British and the Australians have both reported the successful maintenance of fertility in frozen semen stored at -79 deg. C. for over two years.[5] =Use of higher glycerol levels and a -20 deg. C. storage temperature.= In 1953, a report from Arkansas suggested that warmer storage temperatures could be used if a high percentage of glycerol were included in the freezing mixture.[7] To test the effectiveness of various glycerol levels on protecting sperm stored at deep-freeze temperatures, glycerol levels of 3.5, 5.5, 7.5, and 9.5 percent were used with portions of 4 semen samples. Survival in the portions frozen and stored at -20 deg. C. was poor compared with the portions reduced and held at -79 deg. C. In a second experiment, 4 samples were subdivided and frozen with a final concentration of 7, 11, 15, and 19 percent glycerol in the semen-diluent mixture. In this trial, poor results were obtained at -20 deg. C. except that glycerol at a level of 19 percent protected the sperm more effectively than at lower levels. Maximal survival at -79 deg. C. was obtained at the 7-percent glycerol level. A final trial was run, using glycerol levels of 7, 11, 15, 19, 23, 27, and 31 percent. The percentages of motile sperm present after storage at -79 deg. C. and -20 deg. C. are shown in Table 14. Table 14.--Effect of Glycerol Level and Storage Temperature on Freezability of Semen (Average of 8 ejaculates)