Provision of nutrient allowances for sows necessitates a view of requirements for growth and reproductive efficacy, and a method to predict response to the nutrients provided. An experiment involving 102 Large White x Landrace sows given a diet of 13.2MJ DE and 162g CP per kg over 4 parities has been used to derive some of the necessary model components. If live weight and fatness at first conception are 125kg and 14mm P2 respectively, and maternal body weight at term parity 4 is 250kg, and when weaning to oestrus interval (days) = 27(±5) - 1.3(±0.4)P2(mm) at weaning: then the equations, change in P2(mm) in 28-day lactation = -0.28(±0.82) - 0.27(±0.03)P2 at parturition +0.037(±0.003) lactation feed intake - 0.50(±0.06) number of piglets at weaning, and change in maternal live weight (kg) in 28-day lactation = -3.8(±4.6) - 0.15(±0.02) maternal weight at parturition + 0.36 (±0.02) lactation feed intake - 3.3(±0.3) number of piglets at weaning, can be used to predict fat and live weight changes in lactation.