Effects of androgens on the hypo-calcemic response of rats to porcine thyrocalci-tonin were examined. Thyrocalcitonin (0.02 ∼0.08 MRC U) administered sc induced 60 min thereafter a more pronounced hypocalcemia in the male sham-operated than in the male cas-trated animals. In the male, the hypocalcemic response was markedly enhanced by prior trea-tment with testosterone propionate (0.1∼1.0 mg) for 3 days. The same treatments provoked lesser changes of similar type in the female spayed as well as in the female sham-operated animals. Endogenous androgens and exogenous testoster-one propionate caused inconsistent alterations in the basal levels of serum calcium and phosphate. The sensitization of animals to thyrocalcitonin could not be accounted for by their effects on serum phosphate concentration. (Endocrinology87: 421, 1970)