The dissertation begins with a general introduction and a review of the literature on acute fascioliasis in sheep. The various techniques used in this study are outlined, with emphasis on those modified during the work. These include the techniques used in producing the metacercariae, haematological examinations, radio-active isotope studies, liver function tests and serum bio-chemistry. A semi-quantitative precipitin test using living newly excysted flukes is described. The development of the serological response in experimental infections was followed using this test. A positive reaction occurred by two weeks after infection and persisted throughout. The observations made in the course of three experiments on the pathogenesis of acute fascioliasis in sheep are described. There was considerable individual variation but consistent increases occurred in serum glutamic dehydrogenase, serum glutamic oxalacetic transaminase, serum globulin and hence in total serum protein, and in eosinophil counts. Some sheep died before any flukes entered the bile ducts and without any evidence of major haemorrhage. The radio-tracer studies suggested that there is a relative dyshaemopoiea and possibly a terminal haemolytic effect in the severe acute infection. It was shown that the increase in globulins during infection occurred in the beta/gamma globulin components on agar-gel electrophoresis and in the second peak on Sephadex G-200.