Ammonia volatilization following the spreading of animal slurries on lands contributes to reduce the value of slurry as a source of nitrogen and can have repercussions on the wider environment. Field and laboratory studies were carried out to specify (i) the magnitude of ammonia emissions and (ii) the factors that influence the process of ammonia volatilization. Using a wind tunnel system, percentages of NH3 volatilization ranging from 5 to 75% of the ammoniacal-N applied in pig (mainly) and cow slurries were measured during field experiments. On average, 75% of the total loss in all experiments (18) occured within the first 15 hours following slurry application. Ammonia loss, measured during winter and autumn experiments were more than two times lower than loss from spring and summer esperiments. Small volatilization chambers, containing the soil and slurry samples, were used during laboratory experiments. A relation between NH3 volatilization and the slurry volatil fatty acids content was clearly observed. Results from the current work indicate that, in agricultural situations commonly found in Brittany, NH3 volatilization is an important way of nitrogen loss which affect the efficiency of nitrogen recycling from the spreading of animal slurries.