1. Plasma levels of dopa, dopamine, noradrenaline and adrenaline were measured in blood sampled from the renal vein and from the aorta at the origin of the renal artery in six anaesthetized greyhound dogs.
2. Plasma levels of dopa and adrenaline were lower in the renal vein than in the artery, indicating net extraction by the kidney, whereas both noradrenaline and dopamine were secreted into the circulation.
3. Similar measurements of noradrenaline and dopamine in arterial and venous blood were made in a separate group of seven dogs before and during stimulation of the renal nerves and after renal denervation.
4. Stimulation of the renal nerves caused secretion of noradrenaline and of dopamine into the renal vein. After denervation there was no evidence of secretion of noradrenaline into the circulation, whereas dopamine secretion continued, but at a reduced rate, suggesting an extraneuronal source of dopamine.
5. An imbalance of activity between noradrenergic sodium-retaining nerves and an opposing dopamine-secreting natriuretic system could raise blood pressure.