Previous studies using X-ray computed tomography (CT) of Scottish Blackface hill ewes have shown that carcass fat, internal fat and muscle are depleted during pregnancy and early lactation and deposited during late lactation and the dry period (Lambe et al., 2003a). Muscle weights and proportions throughout the year are positively genetically correlated with total litter weight reared, largely due to increased litter size. Average weights of lambs reared were positively correlated with carcass fat weight or proportion pre-lambing and internal fat weight pre-mating, but negatively correlated with internal fat proportion pre-lambing (Lambe et al., 2005). However, is genetic potential to mobilise and regain increased amounts of tissue during the year positively associated with the ability to produce more and heavier lambs? The aim of this study was to address this question by relating total loss and gain of fat and muscle in ewes during the annual cycle with lamb production traits.