Surgically isolated pig thyroid glands were perfused in situ under controlled conditions with blood, or modified Krebs-Ringer bicarbonate buffer, to which individual gastrointestinal hormones or related compounds were added at a constant rate. The thyroid venous effluent was collected quantitatively and the calcitonin (CT) concentration measured by either bioassay or radioimmunoassay so that the CT secretion rate could be calculated. Porcine secretin (50 pM to 145 nM) did not stimulate CT secretion, in contrast to a significant effect with porcine pancreozymin (1.3–25 nM). Stimulation of CT secretion was also produced by the Cterminal octapeptide of pancreozymin (1.8–17 nM), whereas the N-terminal hexapeptide (70 nM) was inactive. This stimulatory action was reduced by mild oxidation. Cerulein (1–14 nM) and pentagastrin (0.5 μM), which share with pancreozymin the same C-terminal tetrapeptide, also stimulated CT release. (Endocrinology89: 262,1971)