With the increasing interest in the use of fibrous feeds as energy sources for pigs, it is important to establish appropriate methods for determining the digestibility of such feedstuffs. In pig nutrient balance studies, a seven-day adaptation period to a new diet is frequently used. However one week may be insufficient for stability of measurements of non-starch polysaccharide (NSP) digestibility to be achieved, for Giusi-Perrier et al (1989) found that fermentation of high-cellulose diets by pigs increased up to 4 weeks after initial exposure to the diet. The aim of this experiment was to measure the development of NSP, nitrogen and energy balance in growing pigs fed contrasting types of NSP at two levels of inclusion over a period of 6 weeks. The two NSP sources chosen for study were wood cellulose (Solka floC) and plain sugar beet pulp (SBP), which are known to be poorly and highly fermentable respectively (Longland and Low 1988).