Groups of 3 castrated male pigs (45-50 kg), housed in metabolism crates, were offered basal diets alone or with increasing levels of addition of straw, oatfeed or sugar-beet pulp. The excretion of nitrogen in the faeces, expressed as a proportion of nitrogen intake, tended to increase as the level of fibre addition increased, the effect being significant (P < 0.05) for the straw diets. Urinary nitrogen excretion, expressed in the same manner, tended to fall as more fibre was consumed, the effect being significant (P < 0.01) for the oatfeed and sugar-beet pulp diets. Increasing fibre in the ration resulted in a significant (P < 0.01) increase in dietary nitrogen retained for oatfeed diets and a similar trend with the other sources of fibre. However, the practical consequences of this effect are not clear.