We have investigated whether removal of the fetal adrenal glands alters the effect of acute hypoxaemia on the circulating concentrations of noradrenaline, adrenaline, Met-Enkephalin and Met-Enkephalin-Arg6-Phe7 (MERF) in the late gestation sheep fetus. Ewes of 6 adrenalectomized and 9 intact fetuses were subjected to a 30 min period of hypoxaemia and a 30 min period of normoxaemia between 135 and 141 days gestation. Removal of the fetal adrenals abolished the fetal adrenaline response and significantly reduced the fetal noradrenaline response to hypoxaemia. There was no significant increase in circulating Met-Enkephalin during either hypoxaemia or normoxaemia in either the intact and adrenalectomized groups. During basal conditions, plasma concentrations of MERF were significantly greater in the adrenalectomized group (1.64 +/- 0.07 ng/ml) than in the intact fetal sheep (1.04 +/- 0.05 ng/ml). There were significantly greater changes (p < 0.05) in plasma MERF concentrations during hypoxaemia than during normoxaemia in both the intact and adrenalectomized groups. Plasma concentrations of MERF were inversely correlated with arterial PO2 (r = -0.44, p < 0.01) in the adrenalectomized but not the intact group of fetal sheep during hypoxaemia. We have demonstrated therefore that the fetal adrenal is the major source of circulating catecholamines, but not Met-Enkephalin or MERF during hypoxaemia in late gestation. The increase in circulating MERF concentrations after adrenalectomy may reflect compensatory changes in the synthesis and/or secretion of MERF in developing sympathetic neurones.