1. The role of the central nervous system (CNS) in maintaining arterial pressure was investigated in two types of experimental hypertension in rabbits.

2. In rabbits with neurogenic hypertension produced by baroreceptor denervation, anaesthesia followed by injection of clonidine (1 μg/kg) into the cisterna magna lowered mean arterial pressure (MAP) from 100·5 ± 1·5 mmHg to 45·0 ± 2·5 mmHg. In normal animals treated similarly MAP fell from 79·1 ± 2·3 mmHg to 49·1 ± 3·9 mmHg.

3. In renal hypertensive rabbits with cellophane perinephritis, intracisternal clonidine decreased MAP from 117·9 ± 4·7 mmHg to 73·0 ± 4·2 mmHg.

4. Acute ganglion blockade with intravenous pentolinium (3 mg/kg) caused a maximum fall in MAP in renal hypertensive animals to 75·8 ± 5·5 mmHg. Combination of intracisternal clonidine and intravenous pentolinium did not significantly augment this fall.

5. The raised arterial pressure in neurogenic hypertension is mediated by the CNS. In renal hypertension, there is a large contribution from the CNS but there are additional non-neurogenic factors elevating the pressure.

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