Parathyroid (PT) glands from 20-day-old embryonic chicks cultured in a chemically defined medium secreted a stimulator of in vitro bone resorption. This stimulator was presumed to be parathyroid hormone (PTH) because: 1) the in vitro dose response curve was parallel to that obtained with bovine PTH; 2) the activity was eluted on Sephadex G-1--chromatography at a postition similar to that for PTH; and 3) the material produced hypercalcemia in vivo in chicks. The amount of PTH-activity secreted was inversely proportional to the calcium concentration of the medium over the range of 0.75-2.25 mM. The chick PT glands also secreted an inhibitor of PTH-stimulated bone resorption in vitro. This inhibitor was presumed to be calcitonin (CT) because: 1) the in vitro dose-response curve was parallel to that obtained with synthetic salmon CT; 2) the activity was eluted on Sephadex G-50 chromatography at a position similar to that for salmon CT; and 3) the material produced hypocalcemia in vivo in rats. In contrast to what would be expected for CT secretion, the CT-activity was secreted by the PT glands in response to a low, not high calcium concentration. The data suggest that the secretion of avian PTH is similar to that of the mammalian hormone, and that the ultimobranchialectomized chick with an intact parathyroid gland may not be deficient in CT.