To evaluate the response of endocrine cells of the gastric oxyntic mucosa in hypergastrinaemic patients to either antrectomy or treatment with the somatostatin analogue octreotide.(a) Two patients with enterochromaffin-like (ECL) cell carcinoid and chronic atrophic gastritis, treated with antrectomy; (b) four patients with Zollinger-Ellison syndrome, treated with octreotide.Oxyntic endocrine cells were examined by ultrastructural morphometry on full thickness biopsy specimens taken: (a) before and four months after antrectomy, (b) before and after three months' treatment with octreotide 200 micrograms daily.Both treatments induced prompt, significant reduction of gastrinaemia and a significant decrease of the volume density of the whole endocrine cell mass and of the cross sectional area of all nucleated endocrine cell profiles (antrectomy: -38%, p < 0.04 and -31%, p < 0.04, respectively; octreotide: -59%, < 0.007 and -26%, < 0.04, respectively). Assessment of the relative proportion of individual endocrine cell types showed a different response to antrectomy or octreotide. After antrectomy, in fact, only the volume fraction of ECL cells was significantly reduced, from 56.5% to 22.5% (-60%, p < 0.04). After octreotide treatment, in contrast, the proportion of all endocrine cell types remained remarkably constant, showing that all cell types took part in the observed overall decrease.Postantrectomy reduction of oxyntic endocrine cells mostly reflects the withdrawal of the specific trophic stimulus of hypergastrinaemia on ECL cells. In contrast, the inhibitory response to octreotide seems to be exerted on virtually all types of oxyntic endocrine cells, probably reflecting a universal occurrence of somatostatin receptors.