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Calcitonin concentrations were determined by bioassay in hydrochloric extracts of ultimobranchial bodies of chickens aged 3 weeks, 3 months, and 9 months. The results were compared with those obtained with two standard preparations (MRC "Thyroid Calcitonin Research Standard A" and a porcine thyroid house standard). Nine-month-old, hypercalcemic hens showed similar concentrations of calcitonin in their ultimobranchial bodies when compared with the two younger age groups of chickens which had been normocalcemic. The maximal hypocalcemic response was 25%. The degree of hypophosphatemia produced by injection of ultimobranchial extracts from the three age groups was also similar, the maximal response being a 35% decrease from original levels. The concentration of calcitonin in ultimobranchial tissue, when expressed in milliunits per milligram fresh gland (125–180 mU/mg) was comparable to values found by other investigators. It is concluded that the hypercalcemia of egg-laying does not lead to a measurable depletion of calcitonin from ultimobranchial bodies.