The purpose of the study was to compare the secretory activity of thyroid and parathyroid mast cell populations during a rise of the level of calcium regulating hormones in the body. 65 rats were divided into 5 equal groups. The animals were given single intraperitoneal injections of the following agents: the 1st group-parathyroid hormone, 5 units per 100 g of body mass; the 2nd group-trilon B, 0.5 ml of 5% solution; the 3rd group-calcitonin, 1.5 units per 100 g of body mass; the 4th group-calcium gluconate, 0.7 ml of 10% solution; the 5th group-distilled water, 0.7 ml; after that the animals were killed with a 12 min. interval. The index of mast cell degranulation was calculated in the thyroid and parathyroid. The administration of PTH and trilon B caused a significant increase in parathyroid mast cell degranulation 36-120 min. after injection whereas secretory activity of thyroid tissue basophils did not change. Mast cell populations of both glands reacted in a similar way to the administration of calcitonin and calcium gluconate expressed in significant enhancement of degranulation 24-84 min. after injection. The results obtained indicated an important role of thyroid and parathyroid tissue basophils in the implementation of inter- and intraorganic regulatory interactions.