Rescue units of the integrated rescue system are in contact with hazardous substances, especially when they break loose and endanger the health and lives of humans, animals and have a negative impact on the environment. Within the frame of safe storage and manipulation with hazardous substances, the relevant legislation must be observed. The main aim of this work was to evaluate the safety of ammonia´s storage in the enterprise Lovochemie, Plc. when an extraordinary event occurs with the usage of ALOHA simulation program. In the work´s introduction, the theoretical knowledge about the area of storage and manipulation with dangerous substances - legislation, safety regulations, characteristics of ammonia (NH3) and simulation program ALOHA were elaborated. In the mentioned program, simulation of model leakage of hazardous substance NH3 was created in the enterprise Lovochemie, Plc. The core of the work is to simulate the leakage of the hazardous substance through the ALOHA program under the current meteorological conditions. The obtained results were compared with the results of the simulation of ammonia´s leakage held by January 7, 2000. The mentioned simulation was used to create an emergency plan for Lovochemie, Plc. From the modeling achieved results, it was obvious that the biggest difference in both cases was mainly influenced by current meteorological conditions and humidity, which was reflected in the size of the affected area. At the end of the diploma thesis, the modeling results were used to plan preventive measures, focusing on protection of the population and securing of space in the case of a special event.