The effect was investigated of the area of slatted floor on ammonia emission and the excretory and lying behaviour of pigs in rooms for rearing and fattening. Climate-controlled rooms had 25% or 50% of the pen floor area slatted. Ammonia concentrations in the incoming and outgoing air and ventilation rates were measured continuously. The urine-fouled floor area (solid and slatted) was visually assessed. Time and location of urination and location of lying of the fatteners were recorded by video. Mean starting and final weights were 10·5 and 25·2 kg in the rearing rooms and 28·1 kg and 105·8 kg in the fattening rooms. The mean overall ammonia emission was 0·85 g/d per rearing pig and 6·10 g/d per fattening pig. During rearing, the ammonia emission from the 25% slatted rooms was 20% (s.e. 7%;P