Trials are described in which the reactions of 11 sheep infested with T. colubriformis and 11 uninfected controls were studied in detail. The main findings in the acute disease were anorexia, retention of ingesta in the rumen and abomasum, diarrhoea, a severe hypo-albuminaemia and death 16 to 17 days after infestation. In the chronic disease there was a progressive decrease in feed intake and loss of body weight. Plasma albumin concentration decreased with a rise in plasma gamma globulin concentration later in the disease. A drop in packed red cell volume, haemoglobin concentration, red cell count, total volume of circulating erythrocytes and plasma inorganic phosphate was noted. There was a decrease in protein, phosphate and possibly selenium uptake leading to emaciation, muscular and myocardial atrophy and degeneration and eventually death.