Resting and stimulated plasma noradrenaline and adrenaline concentration were compared in 45 females and 45 males of similar age who did not smoke, drink alcohol or caffeine-containing beverages. At rest plasma noradrenaline levels were consistently higher in females and adrenaline levels higher in males. These sex-related differences were maintained after isometric exercise, mental arithmetic and cold pressor testing. Resting noradrenaline concentration was negatively correlated with Quetelet index in males and positively correlated with age in females. These findings, based on precise catecholamine measurements under standardized conditions in subjects of similar age, reveal important sex-related differences which need to be taken into account in assessing sympatho-adrenal activity, particularly in relation to mechanisms in essential hypertension.

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1. Arterial plasma renin activity was significantly elevated in rats with one-kidney, one-clip hypertension of less than 3 weeks duratio~.. 2. Intraperitoneal injection of the anglOtensI~ converting enzyme inhibitor SQ 14225 (captopril) caused a dose-related decrease in systolic blood pressure in hypertensive rats. The lowest dose of captopril used (3·5 mg/kg) inhibited conversion of exogenous angiotensin I and maximally potentiated the depressor response to bradykinin, but failed to restore blood pressure to that of the normotensive controls.3. Removal of the solitary clipped kidney also did not restore blood pressure to normal. Injection of captopril (3·5 mg(kg) 2~h~!er nephrectomy, when no circulatmg renm activity was detectable, lowered blood pressure further in hypertensive but not in similarly nephrectomized controls.4. These results indicate that raised blood pressure in early one-kidney,. one-cli~hyper-tension in the rat cannot be entirely attnbuted to the renin-angiotensin system, even when plasma renin activity is significantly increased.5. This study has also confirmed a hypotensive action of captopril in anephric rats when plasma renin activity is undetectable.