1. For comparable blood pressures, plasma noradrenaline is disproportionately higher in patients with phaeochromocytoma than in those with essential hypertension.
2. The ‘pressor-dose’ of noradrenaline or its reciprocal, the reactivity index, were abnormal in patients with phaeochromocytoma before surgery and made normal by surgical removal of the phaeochromocytoma.
3. There was a significant correlation between blood pressures and the logarithm of plasma noradrenaline concentrations.
4. The higher dose of exogenous noradrenaline required in patients with phaeochromocytoma for a given rise in blood pressure seems to be the expression of the higher levels of endogenous noradrenaline rather than true decreased reactivity.