=Rate, temperature, and method of adding glycerol.= Closely associated with the question of how much glycerol should be added is that of how the additions should be made. Originally it was believed that the glycerol should be added in stages so that changes would occur gradually. However, there would be a saving in time if the entire amount could be added at once. Also, if the glycerol addition could be made soon after the dilution with egg yolk-citrate extender at room temperature, time would be gained in processing the semen for use. Since aging _in vitro_ is known to reduce the fertilizing ability of sperm, every effort should be made to keep the processing time at a minimum. The results of an experiment involving these items, along with that of how much time should be allowed after the additions before freezing (equilibration time), are presented in Table 11. One can see that sperm survived freezing better when the diluted semen was cooled to 4.5 deg. C. before the glycerol was added. The survival at 10 deg. and 15.5 deg. C. was reduced with each rise in temperature. Thus, it appears that cooling to refrigerator temperature (4-5 deg. C.) before adding the glycerol should be a part of the routine procedure. A comparison of the results from adding the glycerol in 5, 3, and 1 equal portions is given also in Table 11. Little difference in survival during freezing was noted between the three rates of addition. Using 3 equal additions resulted in slightly better results, but the advantage was not statistically significant. While little difference was evident from adding the glycerol in 3 portions as compared to 1, many still use 3 additions in the hope of obtaining a slightly better sperm survival. In fact, some have gone to a procedure of adding the glycerol dropwise with constant gentle agitation. This method has not been tested in this laboratory.