The ultrastructure of the parathyroid glands in 4-week-old white leghorn chicks has been examined. The gland consists of chief cells arranged in diffuse cords or solid masses. The chief cells vary in electron density. The rough-surfaced endoplasmic reticulum is invariably moderately to well developed and consists of cisternae arranged in well-ordered patterns or distributed randomly. The Golgi complex consists of cisternae arranged in concentric layers, smooth-surfaced and coated vesicles. Cisternae of the Golgi complex arranged in circular and tortuous profiles are also observed. The chief cells contain occasional membrane-limited secretory granules 150–350 nm in diameter and with contents which vary in electron density. Secretory granules are distributed randomly in the cytoplasm and along with the plasma membrane. Images of fusion of coated vesicles with the plasma membrane are sometimes observed. The cytoplasm also contains round- to rod-shaped mitochondria, lysosome-like structures and occasional lipid bodies. Dark cells with electron-dense cytoplasm and containing organelles in close proximity are also observed. Morphological evidence of synthetic and secretory activities in the chief cells suggests active parathyroid function in the rapidly growing chick