123I-metaiodobenzylguanidine (MIBG) localizes in adrenergic neurones. A comparison of % washout of MIBG, during two time intervals, from various organs was carried out in seven normal volunteers and 11 patients with phaeochromocytoma and/or paraganglioma pre- and post-treatment using MIBG. % washout of MIBG during the time intervals of 30 min to 4 h were as follows: heart, controls 26.1 +/- 3.8 v. patients 51.5 +/- 9.1, (p less than 0.001), lungs, controls 36.5 +/- 5.3 v. patients 47.3 +/- 5.8, (p less than 0.05), liver, controls 26.5 +/- 5.7 v. patients 53.4 +/- 12.0 (p less than 0.01). Following treatment the % washout of MIBG from these tissues between 30 min and 4 h were similar in both groups of subjects. This observation coincided with the normalization of plasma catecholamine levels in most of the treatment group. There was no significant difference between the treatment and control groups, in the % washout of MIBG from these tissues during the 4 h and 22 h time intervals either before or after treatment. We conclude that quantitative scintigraphy 30 min to 4 h after MIBG administration is a useful technique to evaluate the pathophysiological effects of high circulating catecholamines in patients with phaeochromocytomas and/or paragangliomas.