Removal of the thyroid gland postprandially from rats did not result in a rise in serum calcium even when endogenous parathyroid activity had been stimulated by extended periods of dietary calcium deficiency. However, removal of the thyroid gland, either immediately or 3 hr after injection of a dose of parathyroid hormone (PTH) not hypercalcemic in intact rats, resulted in a marked rise in serum calcium. A postulated transitory rise in serum calcium in intact rats following PTH injection that might serve as a signal to the thyroid gland to increase thyrocalcitonin release was not detected. The results suggested that if this signal occurred it must have been between 30 and 60 min after injection of PTH. The results also suggest that a very small rise in serum calcium may be sufficient to stimulate increased release of thyrocalcitonin even at blood calcium concentrations slightly below normal. Additional studies showed that, just as the hypercalcemic potency of injected PTH is increased by thyroidectomy, the hypocalcemic potency of thyrocalcitonin in rats is greatly increased during the first hour after parathyroidectomy. (Endocrinology86: 406, 1970)