Semen was used of four breeding bulls of the Bulgarian Red breed and the Alpine Brown one. A total of 26 ejaculates were subjected to 344 laboratory investigations. The semen was diluted in a yolk-lactose-glycerine medium and was frozen by the fast method of Nagase-Niwa. Studied were in dynamics some biologic and biochemical indexes, such as: heat resistance at 46 degrees C, percentage of dead and pathologic spermatozoa, intake of oxygen, and release of CO2. The respiratory coefficient was established by the direct method of Umbreit and coll. The survival rate proved better, lower was the number of dead and pathologic spermatozoa in a test medium containing slightly mineralized water. The consumption of oxygen and the amount of the CO2 released by spermatozoa showed a dependable decrease following equilibration and freezing. Similar, however, unreliable were the data of changes observed with the respiratory coefficient. The semen frozen in the new synthetic medium showed higher biologic value and higher fertilizing capacity (12.2 per cent) as compared with the control.