Experiments are described in which eleven sheep were artificially infested and their reactions studied in detail. The main lesion was found to be a marked decrease in plasma albumin concentration with a resultant shrinkage in plasma volume. There was no indication of anaemia. Electrolyte balance and liver and kidney function were unimpaired. Anorexia was the first symptom exhibited, commencing approximately a week after infestation. A characteristic foetid diarrhoea appeared some two weeks later. The efficacy of N-(2'chlor-4-nitrophenyl)-5-chlorsalicylamid in the treatment of this infestation was strikingly demonstrated. The main pathological lesion was pressure necrosis and erosion of the intestinal mucosa caused by the acetabular sphincter of the fluke. In all cases death occurred only when a considerable number of fluke had commenced their forward migration to the rumen. In highly susceptible sheep the percentage of metacercariae recovered at autopsy as immature paramphistomes was greater than the percentage recovered from resistant sheep after reinfestation. Resistance appeared to be stimulated by the presence of fluke in the rumen.