Leptin is an important regulator of appetite and energy expenditure in adulthood, although its role as a nutritional signal in the control of growth and metabolism before birth is poorly understood. This study investigated the effects of leptin on growth, carbohydrate metabolism and insulin signalling in fetal sheep. Crown-rump length-measuring devices and vascular catheters were implanted in 12 sheep fetuses at 105-110 days of gestation (term 145 +/- 2 days). The fetuses were infused i.v. either with saline (0.9% NaCl; n = 6) or recombinant ovine leptin (0.5-1.0 mg kg(-1) day(-1); n = 6) for 5 days from 125 to 130 days when they were humanely killed and tissues collected. Leptin receptor mRNA and protein were expressed in fetal liver, skeletal muscle and perirenal adipose tissue. Throughout infusion, plasma leptin in the leptin-infused fetuses was 3- to 5-fold higher than in the saline-infused fetuses, although plasma concentrations of insulin, glucose, lactate, cortisol, catecholamines and thyroid hormones did not differ between the groups. Leptin infusion did not affect linear skeletal growth or body, placental and organ weights in utero. Hepatic glycogen content and activities of the gluconeogenic enzymes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase in the leptin-infused fetuses were lower than in the saline-infused fetuses by 44, 48 and 36%, respectively; however, there were no differences in hepatic glycogen synthase activity or insulin signalling protein levels. Therefore, before birth, leptin may inhibit endogenous glucose production by the fetal liver when adipose energy stores and transplacental nutrient delivery are sufficient for the metabolic needs of the fetus. These actions of leptin in utero may contribute to the development of neonatal hypoglycaemia in macrosomic babies of diabetic mothers.