The effect of cimetidine on basal and histamine-induced PTH secretion was tested using single cell suspensions obtained from (a) primary parathyroid adenomas, and (b) secondary hyperplastic parathyroid tissue from patients undergoing chronic hemodialysis. The histamine-stimulated hormone secretion was dose-dependent. Cimetidine suppressed both basal and histamine-stimulated hormone secretion. Within the therapeutic range the suppressive effect was identical for adenoma and hyperplasia. Both adenomata and secondary hyperplastic glands showed a histamine H2-receptor-related response. The role of histamine in the pathogenesis of hyperparathyroidism is not quite clear so that the possible benefits of cimetidine for medical treatment of primary and secondary hyperparathyroidism will have to be proven by careful clinical trials.