A fine-structure study of the parathyroid proper and adjacent tissues has been performed in the gerbils. The chief cells were observed in three phases of the secretory cycle. The resting chief cells contained prominent lipid droplets, mitochondria and an atrophied Golgi apparatus in a finely granular cytoplasm. The intermediate-phase chief cells were also rich in lipid but showed an organelle assembly which is much less developed than in the actively secreting cells. The active chief cells possessed a noncisternal rough endoplasmic reticulum, located often in close proximity of the mitochondria. A prominent Golgi apparatus, numerous pro-secretory granules and lysosomes are characteristic of the active cells. The significance of these morphological variations is discussed. Large mitochondria-rich cells, akin to oxyphil cells were observed in the tissue adjacent to the parathyroid proper.