Sainfoin (Onobrychis viciifolia) is a Mediterranean forage rich in tannins, which has been shown to reduce the faecal excretion of nematode parasites in goats, when offered as hay supplement (Paolini et al, 2003). The use of conserved forages for parasite control may be preferred when either the climatic or agronomic conditions are not appropriate for the grazing of such forages. The aim of this experiment was to investigate whether the consumption of sainfoin hay could reduce the viability and fecundity of adult gastrointestinal parasites, when offered to parasitised sheep at different time points during the course of a parasitic challenge.