A variety of inert -additives; sand, kaolin, cement kiln dust, sawdust and cellulose, were studied with respect to their potential use as dietary diluents. Each of the diluents was found to have no effect upon the Apparent Metabolisable Energy (AME) value of a conventional broiler diet, but, with the exception of sand, all depressed chick growth at three weeks of age. Sand, included in conventional diets at levels of up to 60 g /kg, has been shown to have no effect upon the True Metabolisable Energy (TME) values of the diets nor upon the Endogenous Energy Losses (EEL'S) of adult cockerels. The presence of dietary sand has also been shown to significantly improve broiler growth at 56 days of age but feed intakes were increased proportionally and there was, consequently, no improvement in feed conversion efficiency. It was concluded that sand does not act by merely reducing the energy density of a diet. Cellulose has been shown to have no effect upon the TME value of a conventional diet but does depress chick growth. It was concluded that the growth depression is a consequence of reduced feed intakes. It has also been confirmed that cellulose has no energy available for digestion by poultry. The role of the avian caeca in digestion was studied by feeding conventional and sand- and cellulose -diluted diets. It was concluded that the caeca play no role in digestion under normal circumstances but may play a compensatory role by conserving energy under conditions of stress. uar gum was studied for its effects upon chick growth, the metabolisable energy value of a diet, nitrogen retention and fat digestibility. Growth depression was observed at dietary inclusion levels as low as 7.5 g guar gum /kg. It was concluded that the growth depression was a consequence of a reduction in dietary AME, which itself was a result of an overall reduction in absorption of nutrients, the main effect being upon fat digestibility. The TME value of a conventional diet was found to be unaffected by guar gum when measured by the Sibbald bioassay but it was suggested that this was due to guar gum reducing endogenous energy loss.