As part of their investigation of the source of calcitonin, Copp & Henze (1964) perfused one lobe of the thyroid gland in anaesthetized sheep with hypercalcaemic blood but failed to elicit a systemic hypocalcaemic response. On the other hand, in goats Foster, Baghdiantz, Kumar, Slack, Soliman & Maclntyre (1964) found a marked systemic hypocalcaemia during hypercalcaemic perfusion of a thyroid lobe and two parathyroid glands, but found no such response during hypercalcaemic perfusion of one parathyroid gland only. Care (1965), in young pigs, perfused the thyroid only with high calcium blood and observed a profound fall in the systemic plasma calcium concentration. The explanation for the contrast in the response to stimulation of thyrocalcitonin secretion may lie in a difference in age and also perhaps in the species. Copp & Henze (1964) used five adult sheep and five younger Suffolk sheep which weighed about 40 kg.; the ages of the