Three hormones were demonstrated in ultrathin sections of the rat thyroid using immunocytochemical methods with either a PAP complex or a protein A-gold complex as the label. In control rats, calcitonin was found to be present in all parafollicular cells and somatostatin in occasional cells. In rats pretreated with 5-hydroxytryptophan, serotonin was detected in all parafollicular cells as well. In serial ultrathin sections, the three hormones were seen to be localized in the same secretory granules.