A nutrient partitioning framework has been developed to account for host nutrition-parasite interactions in mammals (Coop and Kyriazakis, 1999). The framework puts forward a nutritional basis for the occurrence of the periparturient relaxation of immunity (PPRI) to gastrointestinal parasites in sheep. The PPRI would be expected to occur because it is proposed that the reproductive effort has a higher partial priority than the immune functions when hosts are given access to scarce nutrient resources, such as metabolizable protein (MP). This would imply that i) immune functions towards parasites will benefit from an increased MP supply, and that ii) the degree of the PPRI depends on the level of reproductive effort. We studied these predictions by comparing performance and resistance in parasitized twin- and single-rearing ewes, which were offered increasing amounts of MP.