1. Blood pressure was measured and plasma levels of noradrenaline and adrenaline were determined radioenzymatically under basal conditions and after 10% blood volume reduction in blood drawn through catheters previously implanted in young and adult rats of two different genetically hypertensive strains: the Kyoto strain (SHR) and the Milan strain (MHS), and in their respective controls: Wistar—Kyoto strain (WKY) and Milan normotensive strain (MNS).

2. Under basal conditions no differences were observed between plasma noradrenaline and adrenaline levels in SHR and MHS rats and in the controls, at any age. Haemorrhage produced a greater fall in the blood pressure (P < 0.01) of young and adult hypertensive strains (SHR-MHS) than in WKY and MNS rats, and a greater rise in plasma adrenaline (P < 0.01).

3. These results suggest that: (a) there may be differences in involvement of the sympathetic nervous system in the pathogenesis of hypertension in SHR and MHS rats but not such as to cause differences in plasma catecholamine levels in either young or adult rats; (b) haemorrhage activates the sympatho—adrenal systems more in SHR and MHS rats, than in controls, and the greater percentage fall in blood pressure is probably due to a difference in reflex venoconstriction.

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