Emissions of ammonia from livestock farming are responsible for the acidification and eutrophication of deposited ammonia in the environment. Research into the ammonia emission from livestock houses was carried out in 14 housing types for cattle, pigs and poultry in England, The Netherlands, Denmark and Germany. Concentrations of ammonia and carbon dioxide (the latter for estimating ventilation rates) were measured at seven locations inside and one location outside in four replicates of each housing type over 24 h under summer and winter conditions. Mean concentrations and emissions per housing type per country were estimated together with some variance components. Mean ammonia concentrations were lower than 8 p.p.m. in cattle houses, between 5 and 18 p.p.m. in pig houses and between 5 and 30 p.p.m. in poultry houses. The concentrations of ammonia in a number of pig and poultry houses exceeded the threshold value of 25 p.p.m. and may affect adversely the health of both stockmen and animals. Ammonia emissions from cattle houses (dairy cows, beef and calves) varied between 80 and 2001 mg/h per animal or 315 and 1798 mg/h (500 kg) live weight. Ammonia emissions from pig houses (sows, weaners and finishers) varied between 22 and 1298 mg/h per animal or 649 and 3751 mg/h (500 kg) live weight. Ammonia emissions from poultry houses (laying hens and broilers) varied between 2.1 and 39.4 mg/h per bird or 602 and 10 892 mg/h (500 kg) live weight. The emission rates should be used carefully, because of large variations between countries, between commercial houses and between seasons. Not all variations could be explained in terms of physical and chemical processes involved in the emission of ammonia. A comparison with other Dutch results revealed that the method used in this research for measuring ammonia emission rates produced accurate mean emission rates.