A salmon calcitonin-like material indistinguishable from synthetic salmon calcitonin-(1-32) on high performance liquid chromatography (HPLC) has been recognized in thyroid extracts of normal subjects and of patients with medullary carcinoma. The same peptide was detected in extracts of the periventricular mesencephalic region which included the periventricular dorsal thalamus, the subthalamus and the hypothalamus. Human calcitonin-(1-32)- and carboxyl-terminal adjacent peptide (CCAP)-like components were also found. The content of immunoreactive salmon calcitonin of the periventricular mesencephalic region (n=6) and of normal thyroid glands (n=6) wa:. comparable (mean ± se, 0.34 ± 0.17 ngeq/g wet tissue and 0.39 ± 0.22 ngeq/g, respectively); and the levels were slightly, but not significantly higher in medullary thyroid carcinoma extracts (1.95 ± 0.69 ngeq/g) (P<0.1). Immunoreactive human calcitonin and CCAP occurred in roughly equimolar concentrations. They were lowest in the periventricular mesencephalic region (0.26 ± 0.09 ngeq/g and 0.46 ± 0.10 ngeq/g, respectively), followed by normal thyroid glands (146 ± 26 ngeq/g and 94 ± 19 ngeq/g, respectively), and they were highest in medullary thyroid carcinoma tissue (680 ± 372 μgeq/g and 144 ± 125 μgeq/g, respectively).