1. Plasma noradrenaline concentrations were similar in normotensive and hypertensive outpatients, but were significantly lower in laboratory control subjects.
2. Standing plasma noradrenaline concentrations were similar in all three groups.
3. Urinary vanillyl mandelic acid, catecholamines and metanephrines were also similar in the normotensive and hypertensive groups.
4. Laboratory controls, possibly because of familiarity with the techniques of sphygmomanometry and blood sampling, may attain a ‘basal’ resting level of sympathetic nervous discharge more readily and rapidly than subjects who are unfamiliar with such procedures.
5. After orthostatic stimulation by standing for 2 min, the activity of the sympathetic nervous system, as determined by pulse rate and plasma noradrenaline concentrations, was similar in the three groups, despite the lower starting values in the laboratory staff.
6. The absence of differences in plasma noradrenaline or urinary catecholamine and metabolite concentrations does not support the hypothesis of excessive sympathetic nervous activity in essential hypertension.