The effect of variable ventilation rates on ammonia emission rates from manure was determined through mass flow measurements in a laboratory test unit and the possibility of reducing ammonia emissions by regulating the intensity of the ventilation in cowsheds. The air exchange rate represents an important determinant of ammonia emissions. A 4.5 times increase in ventilation rate causes ammonia emission to increase by 1.7-2.5 times. The effect of ventilation rate on ammonia emission from manure was found to increase under the condition of extremely high airflow rates, i.e. over 250 m3 h-1 m-2. Under the recommended ventilation rate of 105-420 m3 h-1 per cow, ammonia emission will vary from 2016 to 3195 mg h-1, respectively. The ammonia emissions can be significantly reduced in dairy cattle barns by reducing ventilation rates provided the air quality indicators remain in conformity with the requirements. The ammonia emissions can also be reduced in barns by controlling and diverting air flows inside the barns by preventing the clean air from reaching fresh manure as much as possible while reducing the vertical concentration gradient of ammonia.