A case of Zollinger-Ellison syndrome of multiple endocrine neoplasia type 1 (MEN 1) origin with hyperparathyroidism and with a rise in serum gastrin due to an unusual parathyroid "gastrinoma" has been investigated. The patient had multiple endocrine tumours (pituitary and parathyroid), but no evidence of pancreatic or duodenal gastrin-producing neoplasm. Radio-immunoassay, immunohistochemistry and electron microscopy showed gastrin in one parathyroid adenoma. These findings, together with a decrease of gastrinaemia after parathyroidectomy suggest that true gastrin was produced by parathyroid tumour cells and that they themselves may be the origin of the hypergastrinaemia. Our ultrastructural investigation extends these observations and the results are discussed.