Plasma glucagon concentrations were measured in chronically catheterized fetal ponies and their mothers between 260 days of gestation and term (~ 335 days). Fetal [alpha] cell responses to arginine and variations in fetal glycaemia were also examined during late gestation. Immunoreactive glucagon was present in fetal plasma at 260 days of gestation and its concentration in utero increased after 320 days and then again at birth. Maternal plasma glucagon concentrations were higher after 300 days than earlier in gestation but were lower than the corresponding fetal value throughout the period of gestation studied. Fetal [alpha] cells responded rapidly to intravenous arginine infusion but not to changes in the fetal glucose level induced by maternal fasting for 36 h or by intrafetal infusion of glucose. The maximal increment in fetal plasma glucagon in response to arginine occurred at the end of the 5 min infusion and was positively correlated to the basal pre-infusion plasma glucagon concentrations. Fetal plasma glucagon concentrations were unaffected by either hyper- or hypoglycaemia. In contrast, maternal plasma glucagon levels were significantly increased by fasting. These observations indicate that equine pancreatic [alpha] cells are functional in utero but that they are unresponsive to variations in glycaemia until after birth.