Continuous monitoring of ammonia (NH 3 ) emissions from two mechanically ventilated commercial broiler houses located in the southeastern United States was performed during a one-year period over 2005-2006 as a joint effort between Iowa State University and the University of Kentucky. Ammonia concentrations were measured using Innova 1412 photoacoustic NH 3monitors. Ventilation rates in each house were measured continuously by monitoring the building static pressure and operational status of all ventilation fans in conjunction with individual performance curves developed and verified in situ using a Fan Assessment Numeration System (FANS) unit. Expressed in various units, NH 3 emissions from the two broiler houses over the one-year production period were of the following values: a) 35.4 g per bird marketed (77.9 lb per 1,000 birds marketed), including both grow-out (50-54 d per flock) and downtime (12-25 d between flocks) emissions; b) annual (365-d) emission of 4.63 Mg (5.1 US tons) per house, including both grow-outs and downtime; c) maximum grow-out daily emission of 30.6 kg/d-house (67.4 lb/d-house) for one house and 35.5 kg/d-house (78.2 lb/d-house) for the other; d) mean grow-out daily emission of 14.0 ± 9.1 ( S.D.) kg/d-house; e) mean downtime daily emission of 8.8 ± 8.3 kg/d-house. Flocks on new bedding had a lower emission rate of 12.4 ± 9.4 kg/d-house, as compared to 14.5 ± 8.9 kg/d-house for flocks on built-up litter. The NH 3 emission factor of 35.4 g/bird marketed from this study is substantially lower than that cited by US EPA of 100 g/yr-bird (the US EPA yr-bird unit is equivalent to bird marketed).