As ammonia is a polluting gas, the Dutch agricultural sector is committed to developing ammonia-emission reducing techniques. Furthermore, to improve animal welfare, Dutch pig farmers are using more straw bedding, especially for group-housed sows. A group-housing system for sows can be considered to have four different emitting surfaces: the slurry in the pits, the concrete surfaces of slats and of solid floors, and the straw bedding. Elsewhere (Groenestein et al., 2006), laboratory measurements of the volatilisation of ammonia from samples of these surfaces in response to the application of a dose of urine have been described and it was concluded that a urine pool on straw emits less ammonia than a urine pool of the same size on a slatted or solid floor. This implies that the spatial distribution of urinations influences the ammonia emissions from the house and that emission could be reduced by manipulating the surface the sows urinate upon. The objective of the study described here was to develop model simulations that estimate the effect of the size of the urine pool, the distribution of urine pools over the different emitting surfaces, and the size of the emitting surfaces. The aim was to assess whether the results of such simulations in combination with knowledge of the urinating behaviour of the sows could be important in designing the sow house to reduce environmental pollution (Bos et al., 2003). The reference data were from a house with a floor comprising 60% straw bedding, 14% drinking area (slatted floor with pit), 3% waiting area (slatted floor with pit) and 23% alley (solid floor). Simulations were performed to elucidate the effect of the distribution of the urinations over the different surfaces, relating this to the size and distribution of the urine pools, and the area of the surfaces urinated upon. The results were compared with emission data from an entire sow house. When the default settings were a urine production of 7 l/d per sow, a urination frequency of 5 times a day per sow, and urinations distributed evenly over the four emitting surfaces the model estimated the ammonia emission from the entire house as 11.7 g/d per sow, and the relative contributions of the straw bed, the drinking area, the waiting area and the alley as, respectively, 27%, 22%, 9% and 42%. By comparison, the actual emission from the house was 8.7 g/d per sow. Increasing the size of the urine pool from 0.14m² to 1.40m² in the model simulations caused ammonia emission initially to increase from 9.7 to 12.1 g/d per sow when the pool volume was 0.47m². If the pool was bigger, emission fell to 10.6 g/d per sow because, though the larger emitting area increases ammonia emission, the increase is outweighed by the reduction in emission caused by successive, superseding urinations on the same spot. If the entire emitting area was assumed to be straw bedding, the calculated emission from the house was 5.8 g/d per sow. Assuming slatted and concrete floors without straw bedding increased the emission to 16.5 g/d per sow. It is concluded that measures to reduce the ammonia emission from the bedded sow house should be aimed at decreasing the emission from the solid floor and/or allowing more urinations on the straw bed. The model is a useful design tool for achieving emission reduction from group-housing systems for sows with straw bedding. Its predictive power would be improved by inputting data on the actual size of the urine pool and urinating behaviour of sows.