The effects of octreotide in vivo and in vitro on hormone release, in vivo [123I]Tyr3-octreotide scanning, and in vitro [125I]Tyr3-octreotide autoradiography were compared in five patients with endocrine pancreatic tumors. [123I]Tyr3-octreotide scanning localized the primary tumor and/or previously unknown metastases in four of the five patients. The patient with a negative scan had an insulinoma that did not respond to octreotide in vivo. No Tyr3-octreotide-binding sites were subsequently found at autoradiography of the tumor, whereas somatostatin-14 receptors were present at a high density. In parallel, culture studies with the cells prepared from this adenoma showed that insulin release was not affected by octreotide, while both somatostatin-14 and -28 significantly suppressed hormone release. Culture studies of the tumor cells from two gastrinomas showed a dose-dependent inhibition of gastrin release by octreotide. Octreotide exerted direct antiproliferative effects in one of these gastrinomas, which had been shown to be rapidly growing in vivo. Both gastrinomas had specific somatostatin receptors, as measured by in vitro receptor autoradiography. Somatostatin release by the cultured somatostatinoma cells from one of these patients was suppressed by octreotide. In conclusion, 1) the [123I]Tyr3-octreotide scanning procedure is valuable in the localization of primary endocrine pancreatic tumors as well their often clinically not yet recognized metastases; 2) the in vitro detection of somatostatin receptors in those tumors that were also visualized in vivo after injection of [123I] Tyr3-octreotide indicates that the ligand binding to the tumor in vivo indeed represents binding to specific somatostatin receptors; and 3) the parallel between the presence of somatostatin receptors on tumors and in in vivo and in vitro effects of octreotide on hormonal release from these tumors indicate that a positive scan predicts a good suppressive effect of octreotide on hormonal hypersecretion by these tumors.