Expression of acquired immunity to gastrointestinal parasites usually breaks down during the periparturient period, which is characterised by an increased worm burden and nematode egg excretion. It is believed that this breakdown of immunity may have a nutritional basis, and that by reducing nutrient scarcity the lactating animal will be able to reduce her worm burden. Therefore, the aim of this thesis was to carry out four experiments to investigate the potential use of metabolisable protein as an alternative way to control gastrointestinal parasitism in periparturient animals. A lactating rat model was used to address this issue as lactating rats exhibit a breakdown of immunity to the gastrointestinal nematode Nippostrongylus brasiliensis. The first experiment (chapter two) aimed to verify that a reduction in worm burden is indeed related to changes in nutrient supply and not associated with changes in the gut environment. This was achieved by manipulating nutrient (litter) demand whilst nutrient supply was maintained constant. The results showed that the periparturient breakdown of immunity to N. brasiliensis (measured by a reduced worm burden) was sensitive to changes in nutrient demand and that these effects were independent of changes in the gut environment. The second experiment (Chapter Three) tested the effect of increased protein supply or reduced protein demand on the resistance to parasites in lactating rats whilst energy intake was kept constant. Under these conditions effects of protein supply could not be confounded with effects of any nutrient or energy intake. The results supported the view that under a restricted feeding regime, breakdown of immunity to N. brasiliensis (measured by a reduced number of eggs in the colon content) was sensitive to changes in protein scarcity. Following on from this, the next experiment (Chapter Four) assessed the effects of a gradual increase in protein supply on resistance and immune responses to N. brasiliensis in lactating rats. It was shown that as protein contents of the diets progressively increased, the number of worms and eggs present in the colon decreased. Although we found that differences in protein supply affected parasite burden, we found no affects of protein supply on local immune responses. This may have been due to the single sampling point used. Therefore, the objective of the last experiment (Chapter Five) was to assess temporal effects of increased protein supply on resistance and immune responses to N. brasiliensis. In agreement with previous experiments, the results showed that an increase in protein at times of protein scarcity improved resistance to N. brasiliensis, illustrated by a lower number of nematode eggs in the colon. The results also showed that local immune responses such as immunoglobulin levels (IgA, IgE & IgG2a), RMCP II levels and goblet cell counts were affected by differences in protein supply at various time points post secondary infection. The potential application of using a lactating rat as a suitable model to fully understand the underlying immunological basis of relaxation in immunity during the periparturient period and its sensitivity to nutrient scarcity is considered in the General Discussion (Chapter Six).