Experimental studies were carried out on the pathogenesis of Paramphistomum infection of 58 weaned lambs, aged 4 to 12 months, infected with various numbers of adolescaria. The test animals were divided into 5 groups, each group having 2 subgroups. Most of the animals were given the infectious material only once, while some of them were infected three times at 30-day intervals. The animals of the fifth group were infected with 5,000 live worms each. Adolescaria were obtained from experimentally infected snails of the Galba truncatula species. The changes taking place in both the test and the control animals were followed up, concerning the total protein, protein fractions, hemoglobin, red and white blood picture, and the hematocrit values. The changes in the level of Ca, P, and serum enzymes were also studied. The liver, spleen, kidneys, adrenals, periportal and mesenterial lymph nodes duodenum, and small intestine were sampled for histopathologic investigation, the cross-sectioned material being stained with hematoxylin-eosin and after Mallory (for the demonstration of hyalin). The PAS test was used to demonstrate glycogen. It was found that reconstructive changes took place in the infected animals as judged by the changes in the blood picture and the protein fractions, most substantial of which were those concerning the gamma-globulin fractions which rose almost twice by the 90th day. Microscopically, there were degenerative changes in the parenchymal organs, the duodenum, and the rumen as well as considerable reticulo-histiocytic proliferation and eosinophilic infiltration, with nonreactive necrotic foci in the rumen.