Data on blood glucose, ketone, and acetic acid values have been obtained over a period of several years from field cases of pregnancy toxaemia and from cases induced experimentally and their controls. When feed intake is controlled, differences between blood glucose and ketone levels in ewes moderately undernourished in late pregnancy are largely dependent on the number of foetuses. The consistency of these differences often allows accurate diagnosis of multiple and single pregnancies. Differences in blood glucose between undernourished monotocous and polytocous ewes disappear when undernutrition becomes severe or in the early stages of fasting after a period of moderate undernutrition. Differences in blood ketones tend to be maintained even after several days of fasting. The level of blood ketones during fasting depends on the previous level of nutrition of the ewe; blood glucose is unaffected by previous level of nutrition. The frequent presence of persistent normoglycaemia in ewes with pregnancy toxaemia is discussed in the light of data presented; it is concluded that the onset of pregnancy toxaemia often occurs in the presence of normal blood glucose levels. Blood volatile fatty acid (V.F.A.) levels are often high in those cases of pregnancy toxaemia which occur largely as a result of severe under nutrition in late pregnancy, in spite of an almost empty rumen. Only acetic acid was detected in six blood samples examined; this is considered to be of metabolic origin, but there is no consistent relationship with blood ketone level. The usual clinical syndrome of pregnancy toxaemia can be classified as acute or subacute. In the latter, cerebral depression reaches a certain stage of severity which is then maintained, survival is often prolonged, and ewes do not become comatose before death. Cases of pregnancy toxaemia induced by under nutrition alone usually show the acute syndrome. Blood ketones are higher in such cases than in fasted ewes showing no clinical signs; the critical blood ketone level appears to be about 30 mg per 100 ml. Cases induced in previously well-nourished ewes by fasting in association with a severe environmental stress are usually subacute; blood ketones are no higher than in ewes not showing clinical signs and are often considerably below 30 mg per 100 ml. The significance of these observations is discussed.
