Sainfoin is a leguminous, highly nutritive plant with bloat free characteristics, suitable for growing lambs. In addition, sainfoin has been tested for its anthelmintic properties in goats and sheep, because of its high content of plant secondary metabolites (PSM), specifically condensed tannins. However, Athanasiadou et al. (2005) compared the effect of sainfoin vs lucerne on sheep and the animals showed poor performance, manifested as a lower intake and liveweight gain. But on the other hand, the intake of sainfoin resulted in lower daily parasite egg output. The hypotheses of the present study were: (1) The consumption of sainfoin hay in lambs continuously infected with T. colubriformis will result in negative effects on the parasites, measured through levels of faecal egg counts (FEC). (2) The parasite infection will affect grass hay intake more than sainfoin hay intake, because the PSM content will allow animals to cope better with the parasitic infection. (3) The use of an adaptation period to sainfoin hay prior to infection is expected to improve the parasite control of growing lambs continuously infected with T. colubriformis, through the establishment of a hostile environment for the incoming larvae and also will ameliorate the possible negative effects of PSM contained in sainfoin on the performance of the sheep.