Low birthweight is associated with postnatal physiologic changes, including impaired glucose tolerance and increased cortisol secretion, which may predispose to disease in adulthood. Twins are born lighter than singletons, but there are conflicting data regarding the association between birthweight and postnatal physiology in twins. We studied glucose tolerance and ACTH and cortisol responses to a combined corticotrophin releasing hormone and arginine vasopressin (CRH+AVP) challenge in post-pubertal female twin (n=7 twin pairs) and singleton (n=13) sheep from the same flock. There were no differences in glucose tolerance between twins and singletons and no association with birthweight. Twins had a greater ACTH (P0.05), but not cortisol, response to CRH+AVP than singletons. ACTH area under the curve (AUC) was inversely related to birthweight in both singletons (R²=0.31, P=0.05; -8,311 (3,736) pg.min.ml^-1.kg^-1) and twins (R²=0.49); in twins, this was due to the within-twin pair rather than the between-twin pair coefficient in the regression analysis (P=0.02, -26,856 (9,806) pg.min.ml^-1.kg^-1 vs P=0.1, 8,619 (4,950) pg.min.ml^-1.kg^-1). We conclude that the reduced fetal growth in twins has postnatal consequences for HPA function, and that this is determined by factors specific to the fetus (within-twin pair) rather than by shared maternal factors (between-twin pair). Studies investigating the associations between fetal growth and postnatal outcomes in twins benefit from an appropriate singleton control group and from analyses evaluating the contribution from both between- and within-pair coefficients in twins.