It is generally considered that increased digesta viscosity is one mode of action by which antinutritional factors such as β-glucans in barley and arabinoxylans in wheat reduce digestibility of nutrients in broilers. The increased viscosity interferes, possibly, with the activity of digestive enzymes and subsequent movement of nutrients prior to absorption, therefore ultimately depressing growth rate and feed conversion. Experimental diets are usually fed for approximately three weeks prior to evaluation of gut viscosity. However viscosity may change considerably during and after this time to a degree which may be influenced by the feeding regime employed. Accordingly, the current series of experiments was designed to identify the pattern of change of gut viscosity as influenced by age of bird (15 to 45 days) and feeding regime when diets based on wheat and barley were offered.