The distribution of ten oxidative enzymes was studied in a parathyroid adenoma and in seven surgical biopsies of human parathyroid gland, using histochemical techniques. An intense enzymatic activity particularly of cytochrome oxidase, DPN and TPN diaphorase and of succinic dehydrogenase was found in the oxyphil cells contrasting with the weaker reaction observed in the chief cells. These results give support to the hypothesis that the oxyphil cells are not degenerated as commonly believed but metabolically active.