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Calcitonin (CT) was assayed in the plasma of chick embryos at eight stages of development from day 17 of incubation to day 1 after hatching by a rat bioassay. The concentration of CT rose steadily from 1060 mu./l on day 17 to 4700 mu./l at the stage when pulmonary respiration was initiated on day 20. Nine hours later, when the embryos 'pipped' the shells, the concentration of the hormone more than doubled to reach a peak of 11000 mu./l. At hatching the level had fallen to 1400 mu./l and 24 h later it was undetectable (< 500 mu./l). The physiological significance of these changes is discussed in relation to the development of the embryo.