Eperythrozoon ovis infected sheep have low venous blood glucose levels and correspondingly increased blood lactic acid levels as compared with control sheep. Acid-base studies showed that these changes were accompanied by significant falls in venous pH, and standard bicarbonate as well as a negative base excess. All these changes were considered to result from the increased alvcolytic activity of infected erythrocytes. The acidosis and hypoglycaemia associated with E. ovis infection, while not having any apparent effect on young, well-fed sheep, could be potentially serious in pregnant ewes and in sheep on a low plane of nutrition.