The clinical value of a clonidine suppression test was assessed in three different groups: I. 14 normotensive, healthy people: II. 34 patients with "neurogenic" hypertension: III. 10 patients with pheochromocytoma confirmed at operation. After clonidine (300 mg by mouth) blood pressure (BP) and circulating plasma catecholamines (CA) were reduced in groups I and II. The maximum fall in mean BP was 25/9 mmHg in group I and 43/24 mmHg in group II; the corresponding fall in patients with pheochromocytoma was 19/0 mmHg. The mean concentrations of circulating noradrenaline (NA) were significantly reduced in groups I and II, maximum suppression occurring 2-3 h after giving clonidine. In group I the mean reductions in NA were from 1.3 (SEM 0.2) to 0.6 (0.06) nmol/l (p less than 0.01), and in group II from 2.6 (0.3) to 0.7 (0.05 nmol/l (p less than 0.2); plasma concentrations of A were very low (less than 0.01 nmol/l) in both groups, and remained unchanged throughout the test. In contrast, in the 10 patients with pheochromocytoma the mean basal plasma CA concentrations exceeded our upper reference limit NA 7.7 (2.2 and A 4.0 (1.7 nmol/l), and remained high or even rose during the test. Four patients in this group with only marginally elevated basal plasma CA showed the same pattern of response during clonidine administration. After successful removal of the catecholamine-secreting tumour all patients achieved a normal BP. The plasma CA was suppressed into the normal range when the clonidine test was repeated 1-2 weeks postoperatively. No serious adverse effects were observed.(ABSTRACT TRUNCATED AT 250 WORDS)