A dynamic model was developed to simulate the ammonia volatilization from pig housing with partially slatted floors, where no litter is used. Simulated ammonia emission levels were compared with measured levels for 1 day in each 3-week period during two fattening periods of 15 weeks (one in winter and one in summer) in each of three fattening compartments. The overall mean ammonia emission measured was 6.84 g d-1 pig-1, while the mean overall simulated emission was 6.36 g d-1 pig-1, with 1.96 g d-1 pig-1 simulated to volatilize from the floor and 4.40 g d-1 pig-1 from the slurry pit. The mean relative difference between the daily simulated and measured ammonia emissions was 16.9%. This was 15.0% at the low and moderate emission levels (< 9 g d-1 pig-1) and 27.9% at the high emission levels (> 9 g d-1 pig-1). Simulated effects of different factors corresponded satisfactorily with the measured effects reported in the literature. It is concluded that the ammonia emission from housing for fattening pigs in which the floors are partially slatted can be reasonably well predicted at the low and moderate levels of emission, but is poorly predicted at high emission levels when the solid pen floor is severely fouled at high ambient temperatures.