This article reports two years of quality-assured measurements of concentrations and baseline emissions of ammonia (NH3) at two tunnel-ventilated high-rise houses (houses 3 and 4) located at an egg production facility in North Carolina. The study was conducted as part of the National Air Emissions Monitoring Study (NAEMS). The inside NH3 concentrations, as represented by the exhaust air, were characterized by significant diurnal and seasonal variations. The lowest exhaust concentrations (17.0 +/- 14.0 ppm in house 3; 15.8 +/- 13.0 ppm in house 4) were observed in early afternoon (i.e., 2:00 p.m. to 3:00 p.m) at the maximum diurnal ventilation rate. The highest concentrations (29.5 +/- 22.7 ppm in house 3; 28.8 +/- 22.2 ppm in house 4) occurred in early morning (i.e., 2:00 a.m. to 3:00 a.m.) when the ventilation rate was the lowest. Similarly, the concentrations were lowest during summer and maximum ventilation rates, and highest during winter and minimum ventilation rates. The average NH3 concentrations were 0.7, 22.9, and 20.7 ppm for inlet air and the exhausts of houses 3 and 4, respectively. The average daily mean NH3 emission rate of house 3 (fully occupied and active) was 0.599 +/- 0.200 g d(-1) hen(-1) (197 +/- 66.3 g d(-1) AU(-1), 18.2 +/- 6.04 g d(-1) m(-2)) at an average ambient temperature of 16.7 degrees C and that of house 4 was 0.600 +/- 0.250 g d(-1) hen(-1) (197 +/- 82.3 g d(-1) AU(-1), 18.2 +/- 7.53 g d(-1) m(-2)) at an average ambient temperature of 16.3 degrees C. Ammonia emission rates exhibited less daily and seasonal variation than inside NH3 concentrations. Slightly elevated NH3 emission rates were observed in early afternoon (12:00 noon to 2:00 p.m), when house temperatures were relatively high. No significant differences in emissions between summer and winter were observed. Factors significantly affecting hen-specific NH3 emissions included house ventilation rate, ambient and exhaust air temperatures, exhaust air humidity ratio, hen population, hen activity, feed and water consumption rates, and manure accumulation time. Among all these factors, house exhaust temperature had the greatest effect on NH3 emission rate, followed by manure accumulation time.