Recent estimates of the energy requirements of the pregnant ewe, based on either the amounts of food required to maintain 3-hydroxy-butyrate concentrations in plasma below 0.7 mmol per litre or to prevent the loss of energy from the ewe's body suggest that current recommended feeding standards would result in a fairly severe degree of undernutrition. There is now evidence to suggest that the efficiency of utilisation of metabolisable (ME) energy for fetal growth is positively related to the ME concentration of the diet. During early lactation ewes seldom achieve the high energy intakes (25 to 30 MJ of ME) necessary for the production of the milk (3.5 kg per day) required to achieve desirable total lamb growth rates of 600 to 700 g per day for twins. In this situation the provision of dietary protein supplements which in part escape degradation in the rumen enables the ewe to augment the energy deficit in its diet by utilising body fat.