The thesis consists of six reviews and a dissertation on the research project. In Chapter I various aspects of the epidemiology and pathogenesis of haemonchosis are reviewed. The disease is highly seasonal being more prevalent during the warm humid months, although chronic infections can occur almost all the year round. The parasites suck blood causing haemorrhage into the host's abomasum and the pathogenesis of the disease depends on this blood loss. Factors such as age, breed, haemoglobin type and nutritional status of the host influence the pathogenesis of the disease. The pathogenesis of bovine trypanosomiasis which depends on factors such as anaemia, pathological lesions and immunodepression is described in Chapter II. Although the anaemia is classified as haemolytic, the precise mechanism appears multi-factorial. In Chapter III the use of radiochromium (51Cr) and radioiron (59Fe) in studying the mechanism of anaemias of parasitic infections is examined. Red cells labelled with Cr provide information regarding the contribution made by haemodilution and increased erythrocyte breakdown while 59Fe may be used to monitor the animals' capacity to synthesise red cells. In the study of albumin metabolism in parasitised animals, radioiodine (125I pr 131I) is often employed (Chapter IV). 125I pr 131I-labelled polyvinylpyttolidone (PVP) or 51Cr (as 51crcl3) are the isotopes of choice in providing information regarding the aetiology of gastrointestinal albumin losses in parasitic infections. In Chapter V most parasitic infections are shown to upset the nutritional status of the host through depression of food intake, disturbances of post-absorptive N and energy metabolism and poor feed conversion efficiency. Adequately fed hosts develop immunity earlier which also persists longer than their poorly fed counterparts. In Chapter VI the limited value of conventional methods of control of haemonchosis and trypanosomiasis is highlighted. Similarly, the limited success of vaccination trials is also discussed. In Chapter VII the results of a project carried out to determine the effects of different levels of protein intake on the pathophysiology of acute haemonchosis and on the self-cure phenomenon are presented. Animals on the high protein diet performed better than their poorly fed counterparts as judged by a variety of parasitological, haematological and radioisotopic criteria. The better performance of this group was concluded to reflect a better immune response against the parasite. The nature of this immune response was not determined but was thought to act either against parasite establishment or through suppression of egg laying capability of the female worms. A slight fall in total egg production was observed following reinfection. However pathophysiological changes following reinfection were as severe as recorded following primary infection with the low protein group being more severely affected