It has been proposed that maternal nutrient restriction may alter the functional development of the adipocyte, and the synthesis and secretion of the adipocyte derived hormone, leptin, before birth. We have investigated the effects of restricted periconceptional undernutrition and/or restricted gestational nutrition on fetal plasma leptin concentrations and fetal adiposity in late gestation. There was no effect of either restricted periconceptional or gestational nutrition on maternal or fetal plasma leptin concentrations in singleton or twin pregnancies during late gestation. In ewes carrying twins, but not singletons, maternal plasma leptin concentrations in late gestation were directly related to the change in ewe weight which occurred during the 60 days before mating (maternal leptin = 0.9 (change in ewe weight) + 7.8; r = 0.6, P < 0.05). In twin, but not singleton pregnancies, there was also a significant relationship between maternal and fetal leptin concentrations (maternal leptin = 0.5 fetal leptin + 4.2, r=0.63, P < 0.005). The relative mass of perirenal fat was also significantly increased in twin fetal sheep in the Control-Restricted group (6.0 ± 0.5) when compared to the other nutritional groups (Control-Control: 4.1 ± 0.4; Restricted-Restricted: 4.4 ± 0.4; Restricted-Control: 4.3 ± 0.3). In conclusion, the impact of maternal undernutrition on maternal plasma leptin concentrations during late gestation is dependent on fetal number. Furthermore, we have found that there is an increased fetal adiposity in the twins of ewes which experienced restricted nutrition throughout gestation and this may be important in the programming of postnatal adiposity.