1. During a 2 min period of submersion of normal ducks, sciatic artery blood flow fell to 10 +/- 1.5% control and carotid artery blood flow was reduced to 71 +/- 7% control. Mean arterial blood pressure (M.A.P.), however, was maintained at 83 +/- 3.5% of control. The whole animal showed a constrictor response during submersion, with the sciatic vascular bed showing average constriction. Both resistance to flow and yield pressure increased in the sciatic bed but changed little in the carotid bed. After 1 min submersion P(a,O2) was 52 +/- 1 mm Hg.2. Upon emersion, as soon as ventilation commenced, the whole animal showed a dilator response. The carotid bed exhibited marked vasodilatation whereas the sciatic bed returned to its control level.3. After alpha-receptor blockade, ducks were submerged for 1 min. During this time M.A.P. fell to 64 +/- 5.6% of control and heart rate was reduced to 49 +/- 8.3% of control. Blood flow through the sciatic and carotid arteries also fell to values of 41 +/- 6.9% of control and 91 +/- 13% of control respectively. There was little change in either resistance to flow or yield pressure in the sciatic bed compared to normal ducks, and the carotid bed showed reductions in resistance to flow and yield pressure during submersion. P(a,O2) after 1 min under water was 41 +/- 1.1 mm Hg.4. beta-receptor blockade had no effect on any of the measured variables during submersion. Upon surfacing, however, although the whole animal response was one of dilatation, the carotid bed was less dilated than in normal ducks at this time and the sciatic bed was more constricted.5. Injection of atropine not only abolished the bradycardia during submersion but also caused a rise in M.A.P. and sciatic blood flow during the period under water. After 1 min submersion P(a,O2) was 30 +/- 1.2 mm Hg.6. It is concluded that stimulation of adrenergic alpha-receptors is responsible for the increase in resistance to flow through the sciatic artery and the maintenance of blood pressure during submersion in the normal animals. This selective constrictor activity and the resulting ischaemia is important in the maintenance of P(a,O2) during submersion. Adrenergic beta-receptors (cardiac and/or peripheral) are involved, to a small extent, in the blood pressure and blood flow changes that occur when ventilation commences upon emersion.