Plasma calcitonin levels have been determined by bioassay in Japanese quail and studied with respect to age and sex, and to calcium and magnesium challenge. The plasma calcitonin was assayed by the sensitive rat method of Sturtridge and Kumar, either directly or following Sephadex treatment by the method of Kenny. The plasma calcitonin levels were high, relative to mammalian levels, in both sexes and at all ages studied (1–8 months). The study revealed an interesting relationship between plasma calcitonin levels and se:xual status. Immature males and both immature and mature females had similar levels (mean levels of 328, 396, and 264 MRC μU/ml, respectively). Mature males (2–6 months) on the other hand had much higher levels (mean level of 992 MRC μU/ml). This phenomenon could not be attributed to differences in plasma ionic calcium levels as the latter were higher in females than in males of the same age. Calcium challenge (100 mg Ca/kg, ip) increased the plasma calcitonin levels several—fold 30 min after administration to both mature and immature male quail. Magnesium (20 mg Mg/kg, ip) failed to elicit any such response. It is concluded that plasma calcitonin levels in Japanese quail are high, relative to those in mammals, at all ages and in both sexes; they are even higher in mature male quail.(Endocrinology92: 1754, 1973)