1. Plasma noradrenaline concentrations and platelet α-receptor properties were measured in 14 normal subjects, five phaeochromocytoma patients and nine patients with multiple system atrophy and sympathetic nervous system degeneration (MSA).
2. Plasma noradrenaline concentrations in phaeochromocytoma were 10- to 20-times greater than those of normal subjects and one-fifth to one-half of normal in MSA patients.
3. Platelet α-receptor numbers were increased in MSA and decreased in phaeochromocytoma compared with normal subjects. There was no change in platelet α-receptor affinity in either group.
4. The inverse relationship between α-receptor numbers and plasma noradrenaline concentrations may reflect regulation of α-receptor number by endogenous adrenergic agonist concentrations in man.