Of the metabolic precursors of the catecholamines fixed by the adrenal medulla, 131I marked thyrosine was chosen as a radio-tracer. 6 positive results out of 11 pheochromocytomas confirmed at surgery were obtained in a series of 23 patients. This result is comparable with the results of arteriography but there are more false positives, reflecting the insecurity of this technique which is otherwise quite innocuous.