[female] rats are more sensitive to the anesthetic action of progesterone than [male][male] but this sex difference is obvious only after maturity. The normal endocrine activity of the testis is largely, if not entirely, responsible for this comparative resistance of the [male][male] since castration increases sensitivity in [male][male] but is without effect in [female] rats. Conversely the resistance of castrate oo and oo may be raised by methyltestosterone adm. Treatment with glucose and adrenalin tends to increase, while treatment with insulin and exposure to excessive heat or cold tends to decrease the resistance of the rat to the anesthetic action of progesterone.