The ability of Fasciola hepatica to produce proline was assessed by measuring activity of the proline biosynthesis enzyme ornithine-delta-transaminase, and the arginine-dependent production of proline in vitro. This was related to the changes in bile proline of rats during the course of infection. It was found that the total production of proline and total enzyme activity per fluke increased as the fluke increased in size. However, while the specific enzyme activity remained constant throughout development, there was a preferential increase in proline production relative to protein content of the flukes just prior to their migration into the bile ducts. This early amplification of proline production could provide the stimulus to induce hyperplasia in the bile duct in preparation for fluke entry as proline production relative to protein content of fluke dropped markedly after migration into the bile ducts and never again reached the pre-migration level.