An investigation was conducted into the influence of manure composition on the odour emission rate (OER) and the emission rate of ammonia (NH3) when diets containing 130, 160, 190 and 210 g kg(-1) crude protein (CP) were fed to finishing pigs. A group of four boars and four gilts, housed in environmentally sealed pens, were assigned to each diet for a 23-day experimental period which was replicated three times (n = 3). Ventilation air from each pen was sampled for NH3 and odour, by olfactometry, on four days during the trial period. Simultaneous collections of manure were taken from the surface and base of each pit. The pH and the concentrations of dry matter, total Kjeldahl nitrogen (TKN), total ammoniacal nitrogen (TAN) and volatile fatty acids in the manure were measured. Manure composition differed between samples from the surface and base of the pit (P < 0.05). Reducing dietary CP concentration decreased the emission of NH3 (linear, P < 0.001). The acetic acid:propionic acid ratio in manure samples was correlated to OER (r = 0.79, P < 0.001). There was a quadratic relationship between dietary CP concentration and OER (P < 0.05). OER decreased between 210 g kg(-1) and 160 g kg(-1) CP and increased between 160 g kg(-1) and 130 g kg(-1) CP. In conclusion, reducing dietary crude protein levels could be used effectively to reduce ammonia emissions and OER, although no significant advantage was to be gained in OER from reducing crude protein level below 160 g kg(-1). (C) 2006 Elsevier Ltd. All rights reserved.