Intensive pig production leads to the production of environmental contaminants. These contaminants include organic nitrogen outputs and emissions of ammonia and odour. High rates of nitrogen loading, derived from both organic and gaseous sources, cause eutrophication of nitrogen sensitive ecosystems and acidification of water and soils (Pitcairn et al., 2002). Intensification of livestock production and an increasing urban influx and influence on rural areas has increased objections to odour emissions from pig units (Mackie et al., 1998) and concerns have been raised regarding the impact of odour emissions on human health and well being (Schiffman et al., 1995). It is pertinent, therefore, to employ methods of production that minimise the generation of these contaminants. Previous research has indicated that reducing the concentration of dietary crude protein decreases nitrogen excretion and the volatilisation of NH3 from manure (Canh et al., 1998a; Hayes et al., 2004; Leek et al., 2005). Volatile fatty acids (VFA) in manure reflect bacterial activity and cause a decrease in manure pH that may be helpful in controlling ammonia volatilisation (Canh et al., 1998b). It is reported that the concentration of manure VFA decreases as the concentration of dietary crude protein is reduced (Shriver et al., 2003). Thus, it is necessary to assess whether such an effect on the concentration of manure VFAs, associated with changes in the dietary crude protein concentration, may influence the ammonia emission response. The production of the most pungent and greatest variety of obnoxious smelling compounds emanating from pig production has been attributed to fermentation of nitrogenous material (Hobbs et al., 1997). Fermentation of dietary carbohydrate may also contribute to the formation of odourous metabolites (Hobbs et al., 1996; Miller and Varel, 2003). 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 kgˉ¹ 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. 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. Reducing dietary CP concentration decreased the emission of NH3. The acetic acid:propionic acid ratio in manure samples was correlated to OER. There was a quadratic relationship between dietary CP concentration and OER. OER decreased between 210 g kgˉ¹ and 160 g kgˉ¹ CP and increased between 160 g kgˉ¹ and 130 g kgˉ¹ 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 kgˉ¹.