A brief review is given of the physiological effects of carbon dioxide, methane, ammonia and hydrogen sulphide on pigs. Manure composition is discussed in terms of broad chemical classes such as carbohydrates, proteins and lipids. A tentative composition in terms of molar amounts of carbon, hydrogen, oxygen, nitrogen and sulphur is then derived and used to predict the relative amounts of carbon dioxide, methane, ammonia and hydrogen sulphide that would be generated during gasification of volatile solids. A simple model based on complete-mix anaerobic digesters is proposed for the kinetics of manure degradation in a pit. A mathematical model is then presented that predicts mass transfer between the manure and the air if bulk mean values of concentrations and partial pressures are known. Numerical results are presented from solutions of the mathematical model for climatic and management conditions that are typical in the midwestern states of North America. The model indicates that carbon dioxide from pig respiration will be the component of greatest concern in a pig unit with good internal air movement. The model also suggests that hydrogen sulphide should not accumulate in the manure when there is adequate air movement over the manure. Mass balances on the four components suggest that the model is inadequate with respect to ammonia and that an additional phenomenon, such as urine on the slats, must be invoked to explain measured levels of ammonia in confinement-building atmospheres.