The effect of diphenylhydantoin (DPH) on calcitonin (CT) secretion in the pig was examined. Basal levels of CT were not altered, but the release of CT stimulated by glucagon and by calcium was significantly reduced by prior DPH administration. In spite of suppression of plasma calcitonin, no effect was noted on plasma calcium levels. Further, DPH potentiated the glucagonstimulated elevation of plasma glucose. (Endocrinology92: 330, 1973)