Twenty-four gilts were used at each of two centres in an experiment designed to examine the effects of dietary energy intake during lactation on yield and composition of sows' milk and the growth of their litters. Gestation feed intake was standardized at 2 kg per day supplying 6·28 Meal digestible energy (DE) and 280 g crude protein. The energy levels fed during lactation ranged from 12·2 to 18·25 Meal DE per day for gilts, 12·9 to 19·6 Meal DE per day for second parity and from 13·2 to 20·25 Meal DE per day for third parity sows. Daily crude protein intake and amino acid balance ofthe protein were kept constant.

In the first lactation milk yield and composition and growth of litters were not influenced by dietary energy intake. In the second lactation milk yield was depressed by lower energy intakes and this was reflected in litter weights at 21 though not at 42 days. In the third lactation lower energy intakes seriously depressed milk yield and daily output of milk nutrients. On the lowest level of energy, milk yield was 32% lower at day 24 and 20% lower at day 41 of lactation than on the highest energy level. The corresponding depressions in litter weights were 17·2 and 13·5% at 21 and 42 days respectively.