1. Foetal and maternal plasma catecholamine concentrations were measured during and after hypoxia (mean maternal Pa,02 44mmHg) in chronically catheterized sheep, 118-141 days pregnant. 2. In most foetuses the initial plasma catecholamines were smaller than 0.07 ng/ml. During hypoxia plasma adrenaline and noradrenaline always rose; there was a rise in arterial pressure and a fall in heart rate. 3. The initial catecholamine concentration in the ewes was smaller than 0.05-2.3 ng/ml. During hypoxia there was no consistent change; the maternal plasma concentrations were less than the foetal. 4. Infusion of adrenaline at 0.3 mug kg(-1) min(-1) to the ewe resulted in plasma catecholamine concentrations higher than those observed during hypoxia. There was a rise in heart rate but no consistent change in arterial pressure. 5. Infusion of adrenaline 0.4 mug kg(-1) min(-1) into the foetal jugular vein caused a rise in plasma concentration similar to that seen during hypoxia. There was a rise in heart rate but no significant change in arterial pressure. 6. The half-life of adrenaline and of noradrenaline in the maternal and foetal circulation was 0.25-1 min. There was no evidence of transfer of labelled catecholamine across the placenta.