During two successive years, groups of pregnant ewes and non-pregnant worm-free sheep were given anthelmintic, challenged one week later with 10,000 infective larvae of either Trichostrongylus vitrinus or Ostertagia circumcincta and killed two, four and six weeks after challenge. All of the ewes were killed within three days of their predicted lambing date. There was no annual variation in the response of the ewes to infection with either species. Compared to adult naive animals, ewes expressed resistance to infection with T vitrinus at all times, with significantly lower worm burdens composed largely of inhibited third stage larvae. Ewes challenged with O circumcincta were fully as susceptible as naive animals with regard to the size and stage of development of their worm populations. Mechanisms regulating the numbers of adult O circumcincta were operative in the adult naive animals but not in the ewes.