An ongoing project of monitoring ammonia (NH 3 ) emissions from U.S. layer houses with two different housing and management schemes is reported in this paper. Ammonia and carbon dioxide concentration levels were collected bi-weekly from each house with portable monitoring units (PMUs). There existed substantial seasonal variations in NH 3 emission from the layer houses. Specifically, daily mean NH 3 concentrations ranged from 1 to 7 ppm in the manure belt house and from 9 to 108 ppm in the high-rise house. The NH 3 emission rates averaged 6 mg/hr-hen or 44 g/d-500kg over a 12-month monitoring period (February 2002 – March 2003) for the manure belt house and 44 mg/h-hen or 331 g/d-500kg over a 10-month monitoring period (February – November 2002) for the high-rise house. Ammonia emission rates are higher in summer than in winter, although NH 3 concentration may be much lower in summer.