1. 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.
2. 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.
3. Resting noradrenaline concentration was negatively correlated with Quetelet index in males and positively correlated with age in females.
4. 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.