Preparations of calcitonin from six vertebrate species (man, hog, cow, salmon, cod, chicken) were all shown to be hypocalcemic in young rats. Doses of non—mammalian calcitonins produced longer lasting hypocalcemia than equivalent doses (as determined in the bioassay) of mammalian calcitonin. The hypocalcemic activity of the non—mammalian calcitonins was more persistent than that of their mammalian counterparts during in vitro incubation in rat plasma. The rate of hormone activity loss from the plasma of rats during in vivo tests was more rapid than in the in vitro tests. Non—mammalian calcitonin activity lasted longer than that of mammalian hormones. This relationship was preserved in the nephrectomized rat. (Endocrinology92: 1102, 1973)