The fate of noradrenaline released from sympathetic endings differs from that of noradrenaline secreted directly into the bloodstream. This has been used to establish a single sample test for the exclusion of phaeochromocytoma in patients with borderline elevation of plasma noradrenaline concentration. This test is based on the measurement of the ratio in plasma of noradrenaline to its deaminated metabolite, dihydroxyphenylglycol (DHPG). The latter was shown to reflect mainly nervous release of noradrenaline and its plasma concentration was not increased during intravenous noradrenaline infusion. In seventeen phaeochromocytoma patients the ratio in plasma of noradrenaline to DHPG was greater than 2 (range 2.05-3.57); in nineteen non-phaeochromocytoma patients the reverse was found, the ratio of DHPG to noradrenaline being greater than 2 (range 2.08-2.74). Since DHPG can be measured simultaneously with noradrenaline, measurement of the plasma ratio of these two catechols may prove a simple method of differentiating phaeochromocytoma from nonphaeochromocytoma patients.