1. Anephric and intact rats were submitted sequentially to catecholamine depletion (‘chemical sympathectomy’) vasopressin inhibition and angiotensin blockade whilst blood pressure and plasma levels of each hormone were monitored.
2. Depletion of catecholamines to 15–25% of baseline levels was associated with significant fall of blood pressure. A close correlation existed between these variables.
3. Chemical sympathectomy caused stimulation of plasma vasopressin, which reached excessively high levels in anephric animals. These were inversely correlated with the levels of depleted catecholamines.
4. Vasopressin inhibition after chemical sympathectomy caused profound and lasting hypotension in anephric rats but only a transient small fall in blood pressure in intact rats. Angiotensin blockade after chemical sympathectomy in intact rats caused a transient small fall in blood pressure; subsequent vasopressin inhibition in these rats caused profound lasting hypotension.
5. It is concluded that resting blood pressure is mainly sustained by the sympathetic nervous system, whereas renin and vasopressin are important back-up mechanisms to maintain compromised blood pressure.