Reproductive performance of gilts/sows can be influenced by metabolic state at crucial points in the reproductive cycle and indeed at a crucial age. It has generally been found that moderate to severe protein restriction has no effect on litter size or embryo survival at day 28 (Pharazyn et al, 1991) when given to gilts over the gestation period. However protein quality and quantity can affect reproductive performance in gilts when imposed over the prepubertal period as indicated previously by lower ovulation rate at an induced puberty and poorer expression of a second oestrus (Cia et al, 1996). In the present experiment the effects of two diets differing in lysine:energy ratio on body composition and subsequent oestrus response, ovulation rate and early embryo development in gilts of two different ages were studied.