Propranolol, a beta-adrenergic blocking agent, lowers serum calcium and enhances the hypocalcaemic action of calcitonin in young rats. This effect is not due to an inhibition of parathyroid hormone, since propranolol also produced a fall in serum calcium in thyroparathyroidectomized animals. No changes could be found in serum phosphorus following propranolol administration, which argues against a calcitonin-like mechanism. Propranolol also lowered the serum calcium level in nephrectomized rats, which rules out the kidney as a primary mediator of the hypocalcaemic action. The changes in serum calcium induced by propranolol are most likely due to its membrane stabilizing properties.