Various endocrine-related tumours contain large numbers of high-affinity somatostatin receptors. 123I-labelled tyr-3-octreotide (tyr-3-SMS 201-995, a synthetic derivative of somatostatin) was used to localise such tumours in vivo with a gamma-camera. Positive scans were obtained for two meningiomas, two gastrinomas, and one carcinoid; negative scans were obtained for one insulinoma (in which unlabelled octreotide had no effect on insulin levels), one phaeochromocytoma, one adrenal carcinoma (octreotide had no effect on cortisol levels), and three medullary thyroid carcinomas (octreotide had no effect on calcitonin levels). Thus radioiodinated tyr-3-octreotide can label somatostatin receptors in endocrine-related tumours in vivo and can therefore be used for tumour localisation.