Ammonia emissions were measured in a 100 head grower-finisher operation in the mid-west United States between June – September 1997. This study examined the impact number of hogs (and their respective weight), ventilation rate and temperature (indoor and outdoor) has on the ammonia concentration levels in a mechanically ventilated operation. As well determining appropraite measurement locations in order to gather reliable ammonia emissions data. Throughout the study the average ammonia emission concentration was 5.4 +/- 0.41 ppm, however significant differences were found between pit head space, pit fans and wall fans, as well ammonia concentration in the building was found to be inverselt porportional to the ventilation rate. The avearge daily ammonia emission was 11.2 kg/day, or 122 g/d per 500 kg of body weight, and was found to be directly porportional to the ventilation rate, pig number and indoor temperature.