Groups of rats, infected 7 weeks previously with Fasciola hepatica, together with appropriate control groups, were challenged either orally or intraperitoneally with 30 metacercariae. The mean worm recovery from the previously infected, orally challenged rats was significantly lower than from their respective controls (2.2 +/- 1.1 worms as opposed to 9.0 +/- 2.6). There was no significant difference in mean worm recovery from the previously infected, intraperitoneally challenged rats and their respective controls (5.3 +/- 3.2 worms as opposed to 6.2 +/- 1.9). Livers of the orally challenged group appeared to be largely free from secondary damage but considerable damage was evident in rats which received an intraperitioneal challenge. This evidence supports the view that the gut acts as an important barrier to metacercariae of a challenge infection. In a further experiment, young flukes were recovered from the gut, abdominal cavity and liver of immune and control rats 9, 18, 27, 36 and 45 h after oral challenge. It was found that fewer flukes successfully penetrated the guts of immune rats (3%) than those of uninfected controls (13%), again pointing to the gut as a barrier to metacercariae of a challenge infection. Protective mechanisms that may operate at the level of the gut are discussed.