A clonidine suppression test and the measurement of the catecholamine (noradrenaline and adrenaline) concentration in 24-hour urine were undertaken on 13 patients with benign phaeochromocytoma (PCC), 30 patients with benign hypertension (BHT) and ten healthy, normotensive volunteers. In 11 patients with PCC (85%) the clonidine suppression test gave true-positive results (no significant suppression of initially raised plasma-catecholamine concentration after oral intake of 300 micrograms clonidine). Continuous fall in plasma-catecholamine level after clonidine occurred in two patients with PCC and only moderately elevated initial levels (false-negative results, 15%), as well as in all patients with BHT and all normal controls (true-positive, 100% each). A false-negative result for catecholamine concentration in 24-hour urine was obtained in only one patient (8%). In all others there were either true-positive results (urine concentration greater than 200 micrograms/24 h in 12 patients with PCC, 92%) or true-negative results (urine concentration less than 150 micrograms/24 h in all patients with BHT, 100% each). Compared with direct catecholamine measurement in 24-hour urine, the clonidine suppression test did not fulfil the criteria for further investigations in those patients who had moderately raised plasma-catecholamine levels.